@MonGnoM : Here is the complete list of all hidden specs (bhp and weight only).
And to clarify, they're all in imperial bhp.
The bolden ones are the ones that are completely hidden (i.e. spec's dont show up in the garage even after buying them)
Abarth 1500 Biposto Bertone B.A.T 1 '52 - 73bhp/870kg
Acura DN-X '02 - 1400kg
Acura HSC '04 - 324bhp/1150kg
Acura NSX CONCEPT '13 - 564bhp/1500kg
Acura NSX LM Race Car '91 - 373bhp/1045kg
Alfa Romeo TZ3 Stradale '11 - 1552kg
Amuse S2000 GT1 Turbo - 616bhp/1120kg
Amuse S2000 Street Version '04 - 1190kg
Art Morrison Corvette'60 - 536bhp/1437kg
Audi TT-R '02 - 1080kg
Audi Nuvolari quattro '03 - 1550kg
Audi Pikes Peak quattro '03 - 1900kg
Audi R8 LMS '09 - 528bhp/1300kg
Audi R8 LMS (Team PlayStation) '09 - 528bhp/1300kg
Ayrton Senna DAP Racing Kart #17 '80 - 29bhp/63kg
Ayrton Senna LOTUS 97T '85 - 999bhp/540kg
Ayrton Senna West Surrey Racing, 1983 Ayrton Senna - 164bhp/455kg
BLITZ DUNLOP ER34 SKYLINE '07 - 1252kg
BLITZ ER34 D1 SPEC '04 - 1180kg
BMW Concept 1 Series tii '07 - 304bhp/1520kg
BMW McLaren F1 GTR Race Car '97 - 602bhp/950kg
Cadillac CIEN '02 - 1450kg
Chaparral Chevrolet Chaparral 2X Vision Gran Turismo - 905bhp/450kg
Chevrolet 2010 Dale Earnhardt Jr. #88 AMP Energy/ National Guard CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2010 Jeff Gordon #24 DuPont CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2010 Jimmie Johnson #48 Lowe's CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2010 Juan Montoya #42 Target CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2010 Tony Stewart #14 Office Depot/Old Spice CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2011 Dale Earnhardt Jr. #88 AMP Energy/ NG CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2011 Jeff Gordon #24 Drive to End Hunger CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2011 Jimmie Johnson #48 Lowe's CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2011 Juan Montoya #42 Target CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2011 Tony Stewart #14 Office Depot CHEVROLET IMPALA - 849bhp/1565kg
Chevrolet 2013 Dale Earnhardt Jr. #88 National Guard CHEVROLET SS - 849bhp/1565kg
Chevrolet 2013 Jeff Gordon #24 Drive to End Hunger CHEVROLET SS - 849bhp/1565kg
Chevrolet 2013 Jimmie Johnson #48 Lowe's CHEVROLET SS - 849bhp/1565kg
Chevrolet 2013 Juan Montoya #42 Target CHEVROLET SS - 849bhp/1565kg
Chevrolet 2013 Tony Stewart #14 Bass Pro Shops CHEVROLET SS - 849bhp/1565kg
Chevrolet Camaro LM Race Car - 592bhp
Chevrolet Camaro SS Touring Car '10 - 575bhp/1210kg
Chevrolet Camaro Z28 Race Car '69 - 423bhp/1085kg
Chevrolet Corvette C7 Test Prototype - 455bhp/1500kg
Chevrolet Corvette StingRay Racer Concept '59 - 1150kg
Chevrolet Corvette Stingray (C7) '14 - 1500kg
Chevrolet Corvette Stingray Gran Turismo Concept '13 - 453bhp/1500kg
Chevrolet Corvette Z06 (C6) LM Race Car '06 - 656bhp/1090kg
Chevrolet Corvette ZR1 (C6) LM Race Car '09 - 817bhp/1100kg
Citroën GT by Citroën Race Car - 599bhp/1200kg
Citroën GT by Citroën Road Car - 500bhp/1450kg
Dodge Challenger R/T Race Car '70 - 601bhp/1260kg
Ferrari 458 Italia '09 - 1485kg
Ferrari California '08 - 1630kg
Ferrari F40 '92 - 1352kg
Ferrari SP1 '08 - 495bhp/1450kg
Fisker Karma EcoSport '12 - 398bhp
Ford 2010 AJ Allmendinger #43 Best Buy FORD FUSION - 849bhp/1565kg
Ford 2010 Carl Edwards #99 Aflac FORD FUSION - 849bhp/1565kg
Ford 2011 AJ Allmendinger #43 Best Buy FORD FUSION - 849bhp/1565kg
Ford 2011 Carl Edwards #99 Aflac FORD FUSION - 849bhp/1565kg
Ford 2013 Aric Almirola #43 Farmland FORD FUSION - 849bhp/1565kg
Ford 2013 Carl Edwards #99 Fastenal FORD FUSION - 849bhp/1565kg
Ford Focus RS '02 - 1050kg
Ford GT '06 - 1451kg
Ford GT Concept '02 - 1451kg
Ford GT LM Race Car - 606bhp/1140kg
Ford GT LM Race Car Spec II - 606bhp/1130kg
Ford RS200 Rally Car '85 - 980kg
Gran Turismo Ford GT LM Spec II Test Car - 590bhp/1165kg
Gran Turismo Formula Gran Turismo - 891bhp/550kg
Gran Turismo Amuse/Opera Performance GRAN TURISMO 350Z RS - 491bhp/1450kg
Gran Turismo Racing Kart 100 - 22bhp/80kg
Gran Turismo Racing Kart 100 SPL. - 22bhp/80kg
Gran Turismo Racing Kart 125 - 29bhp/80kg
Gran Turismo Racing Kart 125 SPL. - 29bhp/80kg
Gran Turismo Racing Kart 125 Shifter - 49bhp/80kg
Gran Turismo Racing Kart Junior - 11bhp/80kg
Gran Turismo Red Bull Racing Kart 125 - 29bhp/80kg
Gran Turismo Red Bull X2010 - 1482bhp/545kg
Gran Turismo Red Bull X2010 Prototype - 1482bhp/545kg
Gran Turismo Red Bull X2010 S.Vettel - 1482bhp/545kg
Gran Turismo Red Bull X2011 Prototype - 1556bhp/545kg
Gran Turismo Red Bull X2014 Fan Car - 1193bhp/545kg
Gran Turismo Red Bull X2014 Junior - 246bhp/550kg
Gran Turismo Red Bull X2014 Standard - 798bhp/560kg
High End Performance G37 - 1450kg
Honda CIVIC TYPE R (EK) Touring Car '97 - 247bhp/845kg
Honda CR-Z Touring Car- 199bhp
Honda DUALNOTE '01 - 1400kg
Honda HSC "The 37th Tokyo Motor Show Concept Car" '03 - 324bhp/1150kg
Honda INTEGRA TYPE R (DC5) Touring Car '04 - 298bhp/950kg
Honda INTEGRA TYPE R Touring Car - 305bhp
Honda NSX CONCEPT '13 - 564bhp/1500kg
Honda NSX-R Concept '01 - 1270kg
Honda NSX-R Prototype LM Race Car - 1120bhp
Honda NSX-R Prototype LM Road Car - 1230bhp
Honda S2000 LM Race Car - 315bhp
HPA Motorsports FT565 twin turbo Audi TT - 1338kg
Hudson Mario Andretti's 1948 Hudson - 145bhp/1292kg
Hyundai CLIX '01 - 1320kg
Hyundai Click Type-R '04 - 950kg
Hyundai HCD-6 '01 - 1150kg
Hyundai Tuscani CCS '03 - 1440kg
Infiniti CONCEPT Vision Gran Turismo - 535bhp/1680kg
Infiniti Coupe Concept '06 - 330bhp/1665kg
Infiniti FX45 Concept '02 - 1500kg
Isuzu 4200R '89 - 1500kg
Jay leno Tank Car '03 - 3500kg
Lamborghini Miura P400 Bertone Prototype CN.0706 '67 - 980kg
Lancia STRATOS Rally Car '77 - 880kg
Lexus IS F CCS-R '11 - 1400kg
Lexus IS F Racing Concept '08 - 525bhp/1350kg
Lexus IS F Touring Car '07 - 619bhp/1185kg
Lexus LF-LC GT "Vision Gran Turismo" - 612bhp/1075kg
Lotus Elise 111R Race Car '04 - 261bhp/720kg
Lotus Elise Race Car '96 - 173bhp/645kg
Lotus Evora '09 - 1330kg
Mazda Atenza Concept '01 - 1300kg
Mazda Atenza Touring Car - 295bhp
Mazda Furai '08 - 500bhp/675kg
Mazda KUSABI '03 - 900kg
Mazda LM55 Vision Gran Turismo - 791bhp/830kg
Mazda MX-Crossport '05 - 255bhp/1600kg
Mazda Mazda6 Concept '01 - 1300kg
Mazda Mazda6 Touring Car - 304bhp
Mazda Mazdaspeed Atenza '05 - 1500kg
Mazda RX-7 LM Race Car - 500bhp
Mazda RX-7 Touring Car - 463bhp
Mazda RX-8 Concept (Type-II) '01 - 1200kg
Mazda RX-8 Concept LM Race Car - 500bhp
Mazda Roadster Touring Car - 205bhp
Mercedes-Benz AMG 190 E 2.5 -16 Evolution II '92 - 980kg
Mercedes-Benz AMG Mercedes CLK-LM '98 - 1000kg
Mitsubishi CZ-3 Tarmac '01 - 225bhp/1200kg
Mitsubishi CZ-3 Tarmac Rally Car - 314bhp
Mitsubishi Concept XR-PHEV EVOLUTION Vision Gran Turismo - 502bhp
Mitsubishi FTO Super Touring Car - 423bhp
Mitsubishi Lancer 1600 GSR Rally Car '74 - 825kg
Mitsubishi Lancer Evolution IX GSR Touring Car '05 - 401bhp/1080kg
Mitsubishi Lancer Evolution X Rally Car - 441bhp
Mitsubishi PAJERO Evolution Rally Raid Car '03 - 1825kg
NISMO Fairlady Z S-tune concept by GRANTURISMO (Z33) '02 - 273bhp/1320kg
NISMO Fairlady Z Z-tune (Z33) '03 - 384bhp/1250kg
Nissan 350Z Concept LM Race Car - 499bhp
Nissan CONCEPT 2020 Vision Gran Turismo - 697bhp/1575kg
Nissan Fairlady Z Concept LM Race Car - 608bhp
Nissan GT-R Black Mask '07 - 502bhp/1740kg
Nissan GT-R Concept (Tokyo Motor Show 2001) '01 - 447bhp/1560kg
Nissan GT-R Concept LM Race Car - 616bhp
Nissan GT-R LM Nismo '15 - 604bhp
Nissan mm-R Cup Car '01 - 137bhp/672kg
Nissan GT-R N24 GT Academy '12 - 1700kg
Nissan GT-R Proto '05 - 445bhp/1730kg
Nissan GT-R R35 Touring Car - 654bhp
Nissan OPTION Stream Z '04 - 1440kg
Nissan SILVIA spec-R AERO (S15) Touring Car '02 - 355bhp/1000kg
Nissan SKYLINE GT-R R32 Touring Car - 495bhp
Nissan SKYLINE GT-R R33 Touring Car - 520bhp
Nissan SKYLINE GT-R R34 Touring Car - 555bhp
Nissan Zytek Z11SN Greaves Motorsport '13 - 900kg
Opera Performance 350Z '04 - 345bhp/1340kg
Opera Performance S2000 '04 - 270bhp/930kg
Pagani Zonda LM Race Car - 650bhp
Panoz Esperante GTR-1 Race Car '98 - 741bhp
Pescarolo C60 Hybride - Judd '05 - 900kg
Pescarolo Courage - Judd GV5 '04 - 900kg
Pescarolo Courage C60 - Peugeot '03 - 900kg
Plymouth XNR Ghia Roadster '60 - 1500kg
RE Amemiya FD3S RX-7 - 1240kg
Renault Sport Clio V6 Trophy 24V '00 - 293bhp
Renault Sport Lutecia R.S. Trophy V6 24V '00 - 293bhp
Scion FR-S '12 - 1230kg
Spoon CR-Z - 142bhp/1130kg
Spoon FIT Race Car '03 - 123bhp/990kg
Spoon INTEGRA TYPE R (DC2) '99 - 216bhp/1080kg
SRT Challenger SRT8 Touring Car - 604bhp
SRT Viper GTS-R (Team Oreca) #51 '00 - 587bhp
SRT Viper GTS-R (Team Oreca) #91 '00 - 587bhp
Subaru IMPREZA Rally Car Prototype '01 - 300bhp
Subaru IMPREZA Sedan WRX STI Rally Car '05 - 410bhp/1065kg
Subaru IMPREZA Super Touring Car - 415bhp
Subaru IMPREZA Touring Car - 454bhp
Subaru IMPREZA WRC 2008 - 1230kg
Suzuki CONCEPT-S2 '03 - 176bhp/1000kg
Suzuki Cappuccino (EA21R) Race Car '95 - 91bhp/590kg
Tajima 2012 Monster Sport E-RUNNER Pikes Peak Special - 887bhp/1640kg
Toyota 2010 Brian Vickers #83 Red Bull TOYOTA CAMRY - 849bhp/1565kg
Toyota 2010 Denny Hamlin #11 FedEx TOYOTA CAMRY - 849bhp/1565kg
Toyota 2010 Joey Logano #20 The Home Depot TOYOTA CAMRY - 849bhp/1565kg
Toyota 2010 Kyle Busch #18 M&M'S® TOYOTA CAMRY - 849bhp/1565kg
Toyota 2011 Brian Vickers #83 Red Bull TOYOTA CAMRY - 849bhp/1565kg
Toyota 2011 Denny Hamlin #11 FedEx TOYOTA CAMRY - 849bhp/1565kg
Toyota 2011 Joey Logano #20 The Home Depot TOYOTA CAMRY - 849bhp/1565kg
Toyota 2011 Kyle Busch #18 M&M'S® TOYOTA CAMRY - 849bhp/1565kg
Toyota 2013 Denny Hamlin #11 FedEx TOYOTA CAMRY - 849bhp/1565kg
Toyota 2013 Kyle Busch #18 M&M'S® TOYOTA CAMRY - 849bhp/1565kg
Toyota 2013 Matt Kenseth #20 Dollar General/Home Depot TOYOTA CAMRY - 849bhp/1565kg
Toyota 86 Race Car Base Model '12 - 1230kg
Toyota ALTEZZA Touring Car - 300bhp
Toyota FT-1 - 486bhp/1320kg
Toyota FT-1 graphite - 486bhp/1320kg
Toyota FT-1 Vision Gran Turismo - 614bhp/1000kg
Toyota FT-86 Concept '09 - 227bhp/1000kg
Toyota FT-86 G SPORTS Concept '10 - 298bhp/1000kg
Toyota FT-86II concept '11 - 198bhp/1230kg
Toyota GAZOO Racing TOYOTA 86 #166 '12 - 1230kg
Toyota PRIUS Touring Car - 201bhp
Toyota RSC '01 - 276bhp/1350kg
Toyota RSC Rally Raid Car - 416bhp
Toyota SPRINTER TRUENO GT-APEX (AE86 Shuichi Shigeno Version) '00 - 207bhp
Toyota TS030 Hybrid '12 - 529bhp/900kg
TVR Tuscan Speed 6 Touring Car '00 - 495bhp/885kg
Volkswagen GTI Roadster Vision Gran Turismo - 551bhp/1200kg
Volkswagen GTI Supersport Vision Gran Turismo - 551bhp/1200kg
Volkswagen Golf IV GTI Touring Car '01 - 216bhp/980kg
Volkswagen W12 Nardo '01 - 1300kg