Below are some results I obtained using jamjam's CDGroup utility. Although intended for internal use, I thought they might prove useful for reference purposes.
Conditions were as follows:
Dataset: PSX (all regions)
Discs were regionally grouped where available
Demos were appropriately grouped with full releases
Multiple point releases were grouped
Multi-disc titles were appropriately grouped
Audio tracks were compressed post-split (ape -c5000)
t7z was replaced with 7z 7.25a, 64 bit (-mx9 -m0=LZMA:d1024m:fb273 -mmt2)
Time stamps and file attributes were scrubbed from all of the archives for consistency and cross-compatibility.
All discs conformed to Redump standard
Merged sets adhere to Redump.org naming convention with the following alterations:
Merged set containing 2 or more regions have no regional suffix assigned (those containing only 1 region retain their regional suffix as before)
Multi-disc title within a merged set has the disc label removed
Multi-point release within a merged set has the version label removed
Merged name: USA>Europe>Japan
Of the chosen sample data, 881 discs were grouped down to 197. Each title in the dataset below is suffixed by 3 numbers: combined torrent7z size (MB), cdgroup size (MB) and the ratio between the two (in red).
Summary
Raw size (GB):
489.1
torrent7z size (GB):
257.4
CDGroup size (GB):
80.0
Relative compression:
X-Axis: Merged set # / Y-Axis: Compression ratio relative to t7z
Dataset
3D Lemmings 1214.3 299.5 0.2
3x3 Eyes - Kyuusei Kousyu (Japan) 577.1 353.0 0.6
3x3 Eyes - Tenrinou Genmu (Japan) 654.5 368.3 0.6
A Ressha de Ikou 4 - Evolution Global (Japan) 143.3 83.1 0.6
Ace Combat 2 1179.8 322.4 0.3
Ace Combat 3 - Electrosphere 1987.1 589.8 0.3
Aconcagua (Japan) 365.3 267.3 0.7
Air Combat 1615.8 593.4 0.4
Akagawa Jiro - Yasoukyoku (Japan) 308.8 132.3 0.4
Alien Trilogy 1446.1 244.5 0.2
Alnam no Tsubasa - Shoujin no Sora no Kanata e (Japan) 663.5 451.4 0.7
Alundra 2 - A New Legend Begins 1356.4 763.2 0.6
Alundra 2037.7 252.8 0.1
Animetic Story Game 1 - Card Captor Sakura (Japan) 546.6 387.0 0.7
Ape Escape 676.8 333.3 0.5
Arc the Lad Collection - Arc the Lad II 731.3 316.0 0.4
Arc the Lad Collection - Arc the Lad III 1003.1 425.0 0.4
Arc the Lad Collection - Arc the Lad 416.0 297.4 0.7
Area 51 1094.3 283.1 0.3
Armored Core - Master of Arena 1631.8 260.1 0.2
Armored Core - Project Phantasma 579.0 160.4 0.3
Armored Core 709.8 195.3 0.3
B Senjou no Alice - Alice on Borderlines (Japan) 1199.6 1012.9 0.8
Back Guiner - Yomigaeru Yuusha Tachi - Hishou Hen 'Uragiri no Senjou' (Japan) 591.2 403.0 0.7
Back Guiner - Yomigaeru Yuusha Tachi - Kakusei Hen 'Guiner Tensei' (Japan) 587.3 390.5 0.7
Bounty Sword - Double Edge (Japan) 639.5 297.7 0.5
Brave Fencer Musashi 518.2 232.3 0.4
Breath of Fire III 234.5 53.6 0.2
Breath of Fire IV 466.8 116.7 0.2
Brigandine - Grand Edition (Japan) 821.2 550.6 0.7
Bushido Blade 2 532.4 332.6 0.6
Bushido Blade 734.2 333.8 0.5
Castlevania - Symphony of the Night 1714.5 384.2 0.2
Chibi Chara Game Ginga Eiyuu Densetsu (Reinhart Version) (Japan) 311.8 239.4 0.8
Chocobo's Dungeon 2 474.1 243.7 0.5
Chou Jikuu Yousai Macross - Ai Oboete Imasu ka (Japan) 346.4 302.5 0.9
Chrono Cross 1334.0 412.2 0.3
Crash Bandicoot - Warped 433.3 240.0 0.6
Crash Bandicoot 2 - Cortex Strikes Back 393.6 127.8 0.3
Crash Bandicoot 1714.6 372.7 0.2
Crash Bash 403.0 277.0 0.7
Cross Tantei Monogatari (Japan) 522.2 394.4 0.8
CTR - Crash Team Racing 1120.5 466.0 0.4
Dancing Blade Katte ni Momotenshi II - Tears of Eden (Japan) 840.1 658.2 0.8
Dancing Blade Katte ni Momotenshi! (Japan) 703.0 487.0 0.7
Darkstalkers 3 1263.0 412.6 0.3
Dead or Alive 1263.6 346.5 0.3
Devil Summoner - Soul Hackers (Japan) 489.5 271.5 0.6
Diablo 799.4 290.2 0.4
Die Hard Trilogy 2 - Viva Las Vegas 717.0 316.0 0.4
Die Hard Trilogy 1223.0 318.6 0.3
Dino Crisis 2 1419.4 589.6 0.4
Dino Crisis 1059.1 263.1 0.2
Dokapon! Ikari no Tekken (Japan) 134.1 34.8 0.3
Doki Doki Pretty League - Nekketsu Otome Seishunki (Japan) 1024.0 610.7 0.6
Dragon Ball GT - Final Bout 431.6 100.7 0.2
Dragon Ball Z - Ultimate Battle 22 1217.1 155.5 0.1
Dragon Valor 701.8 112.8 0.2
Dragon Warrior VII 1417.5 421.3 0.3
Driver - You Are the Wheelman 1814.5 658.5 0.4
Eberouge (Japan) 787.0 520.9 0.7
Echo Night 319.8 157.3 0.5
Ehrgeiz - God Bless the Ring 1332.6 422.1 0.3
Einhander 631.2 254.9 0.4
Eithea (Japan) 455.4 325.0 0.7
Enigma (Japan) 817.3 658.7 0.8
Evil Zone 1350.4 726.3 0.5
Final Fantasy Anthology 906.5 277.7 0.3
Final Fantasy Chronicles - Chrono Trigger 764.1 245.3 0.3
Final Fantasy Chronicles - Final Fantasy IV 823.4 178.2 0.2
Final Fantasy IX 11586.6 1204.6 0.1
Final Fantasy Origins 412.6 82.0 0.2
Final Fantasy Tactics 545.3 219.2 0.4
Final Fantasy VII International (Japan) (Disc 4) (Perfect Guide) 398.8 340.0 0.9
Final Fantasy VII 10626.1 1174.3 0.1
Final Fantasy VIII 12411.1 1400.9 0.1
From TV Animation One Piece - Tobidase Kaizokudan! (Japan) 744.9 310.3 0.4
Front Mission 2 (Japan) 249.6 101.0 0.4
Front Mission 3 917.5 267.2 0.3
G-Police 3730.7 1148.5 0.3
G. Darius 1112.5 348.7 0.3
Galaxian 3 754.9 333.0 0.4
Ganbare Goemon - Uchuu Kaizoku Akogingu (Japan) 728.7 353.3 0.5
Global Force - Shin Sentou Kokka (Japan) 271.7 137.5 0.5
Gradius Gaiden (Japan) 587.7 282.4 0.5
Gran Turismo 2 4035.7 1481.0 0.4
Gran Turismo 2094.0 565.2 0.3
Grandia 3182.6 872.4 0.3
Guardian Recall - Shugojuu Shoukan (Japan) 830.1 669.5 0.8
Gundam 0079 - The War for Earth (Japan) 410.8 286.4 0.7
Gungage 327.3 135.1 0.4
Harukaze Sentai V-Force (Japan) 770.3 465.9 0.6
Hellnight 716.1 574.4 0.8
Hot Shots Golf 2 506.4 167.7 0.3
Hot Shots Golf 443.1 122.9 0.3
Jikuu Tantei DD - Maboroshi no Lorelei (Japan) 520.9 359.0 0.7
Jikuu Tantei DD 2 - Hangyaku no Apusararu (Japan) 486.6 387.9 0.8
JoJo's Bizarre Adventure 1145.4 381.4 0.3
Juggernaut 2195.9 974.0 0.4
Jumping Flash! 2 675.3 215.4 0.3
Jumping Flash! 609.7 205.0 0.3
Kidou Senshi Gundam - Gyakushuu no Char (Japan) 547.8 223.1 0.4
Kidou Senshi Z-Gundam (Japan) 1637.0 554.6 0.3
Kindaichi Shounen no Jikenbo 2 - Jigoku Yuuen Satsujin Jiken (Japan) 846.9 680.0 0.8
King's Field II 723.6 272.6 0.4
King's Field 333.2 174.3 0.5
Klonoa - Beach Volleyball 190.0 92.9 0.5
Koudelka 4016.4 1363.7 0.3
Lunar - Silver Star Story Complete 1247.3 782.0 0.6
Maria - Kimitachi ga Umareta Wake (Japan) 801.9 633.5 0.8
Maria 2 - Jutai Kokuchi no Nazo (Japan) 854.8 593.1 0.7
Megaman - Battle & Chase 684.7 288.1 0.4
Megami Ibunroku Persona - Be Your True Mind (Japan) 549.7 210.6 0.4
Meltylancer - Re-inforce (Japan) 811.3 615.5 0.8
Metal Gear Solid - VR Missions 909.9 483.6 0.5
Metal Gear Solid 7287.3 3421.1 0.5
Myst 1290.8 501.2 0.4
Namco Museum Encore (Japan) 588.6 276.8 0.5
NBA in the Zone '98 529.0 208.6 0.4
NBA in the Zone '99 654.5 294.2 0.4
NBA in the Zone 2 458.6 122.3 0.3
NBA in the Zone 2000 363.6 121.8 0.3
NBA in the Zone 640.4 152.8 0.2
Nessa no Hoshi (Japan) 925.0 629.1 0.7
Nightmare Project - Yakata (Japan) 491.1 326.8 0.7
NOeL - La Neige (Japan) 1820.1 480.4 0.3
NOeL - Not Digital (Japan) 1241.2 579.8 0.5
NOeL 3 - Mission on the Line (Japan) 962.8 586.4 0.6
Novastorm 2280.3 698.4 0.3
Option - Tuning Car Battle (Japan) 784.7 281.8 0.4
PaRappa the Rapper 955.5 499.1 0.5
Parasite Eve II 4049.0 624.0 0.2
Parasite Eve 1064.8 393.3 0.4
R4 - Ridge Racer Type 4 989.7 415.5 0.4
Rage Racer 2431.7 466.5 0.2
Resident Evil - Survivor 567.3 150.8 0.3
Resident Evil 2 6884.4 743.7 0.1
Resident Evil 3 - Nemesis 2423.4 194.8 0.1
Resident Evil 3998.9 491.8 0.1
Ridge Racer Revolution 2257.7 596.1 0.3
Ridge Racer 1050.4 284.9 0.3
Rival Schools - United by Fate 1444.2 506.9 0.4
SaGa Frontier 2 785.1 151.5 0.2
SaGa Frontier 313.3 100.3 0.3
Silent Hill 1048.3 244.3 0.2
Sister Princess (Japan) 1352.0 476.8 0.4
Sister Princess 2 (Japan) 2190.4 733.9 0.3
Skydiving Extreme 510.6 237.4 0.5
Sol Divide 173.5 86.1 0.5
Soul Blade 1285.7 335.5 0.3
Star Ixiom 443.4 269.0 0.6
Star Ocean - The Second Story 2241.7 366.4 0.2
Starblade Alpha 1199.7 325.4 0.3
Street Fighter - The Movie 893.9 330.6 0.4
Street Fighter Alpha - Warriors' Dreams 1443.2 403.6 0.3
Street Fighter Alpha 2 868.9 272.0 0.3
Street Fighter Alpha 3 838.3 266.4 0.3
Street Fighter Collection 2 445.9 214.3 0.5
Street Fighter Collection 1567.5 613.0 0.4
Street Fighter EX Plus Alpha 746.0 241.6 0.3
Street Fighter EX2 Plus 561.8 184.8 0.3
Street Fighter II Movie (Japan) 844.9 692.9 0.8
Strikers 1945 II 523.9 224.4 0.4
Suikoden II 1311.6 258.4 0.2
Suikoden 1693.0 278.2 0.2
Tales of Destiny II 1719.7 822.8 0.5
Tales of Fandom Vol. 1 (Japan) 624.6 296.8 0.5
Tantei Jinguji Saburo - Early Collection (Japan) 505.3 238.1 0.5
Tekken 2 1894.5 430.2 0.2
Tekken 3 1851.8 480.0 0.3
Tekken 2266.0 395.9 0.2
Tenchu - Stealth Assassins 1682.0 631.3 0.4
Tenchu 2 - Birth of the Stealth Assassins 1914.2 872.9 0.5
Tennis no Oujisama - Sweat & Tears (Japan) 652.8 298.9 0.5
Theme Park 397.3 120.6 0.3
Thousand Arms 1735.0 695.8 0.4
Threads of Fate 93.2 25.6 0.3
Time Crisis - Project Titan 1117.9 322.6 0.3
Time Crisis 1548.0 371.7 0.2
Tobal 2 (Japan) 769.9 339.2 0.4
Tobal No.1 1266.6 296.2 0.2
Tokimeki Memorial - Forever with You (Japan) 1414.5 441.7 0.3
Tomb Raider - The Last Revelation 2826.6 629.9 0.2
Tomb Raider Chronicles 2473.8 854.0 0.3
Tomb Raider II 3630.7 518.8 0.1
Tomb Raider III - Adventures of Lara Croft 4309.6 533.0 0.1
Tomb Raider 4022.4 479.8 0.1
Um Jammer Lammy 1889.7 719.5 0.4
Vagrant Story 337.7 48.8 0.1
Valkyrie Profile 1954.4 438.9 0.2
Vandal Hearts II 374.7 80.0 0.2
Vandal Hearts 706.6 453.0 0.6
Wild Arms 2 1256.5 297.9 0.2
Wild Arms 1690.9 257.5 0.2
WipEout XL 2262.6 420.6 0.2
WipEout 2078.1 394.7 0.2
Xenogears 1305.1 524.4 0.4