I've lost count of the number of places that I've posted this list, but here it is again. The AI cars for Nurburgring 4hr are a mixture of stock power (e.g. Jaguars) and tuned cars (e.g. Alfa Romeo, Fiat, Lotus, etc.) although there's no way to know if any of them have transmission, braking, suspension or weight-reduction modifications as well.
Car name [Power (HP) from race Preview in square brackets]
Jaguar S-Type R '02 [403]
Jaguar XKR R-Performance '02 [393]
Audi S4 '03 [390]
Audi RS 4 '01 [390]
FPV F6 Typhoon '04 [372]
Ford Mustang SVT Cobra R '00 [370]
Fiat Coupe Turbo Plus '00 [361]
Ford Focus RS '02 [362]
Lancia Delta HF Integrale '91 [363]
Opel Speedster Turbo '00 [362]
Subaru Impeza WRX STi spec C '04 [362]
Alfa Romeo 147 GTA '02 [352]
ASL Garaiya '02 [353]
BMW M3 '04 [349]
BMW M Coupe '98 [352]
Chevrolet Camaro SS '00 [346]
Chevrolet Corvette Grand Sport (C4) '96 [347]
Holden Commodore SS '04 [347]
Honda NSX Type R '02 [350]
Honda NSX Type S '01 [349]
Lotus Elise 111R '04 [347]
Mazda RX-8 Type S '03 [352]
Mercedes CLK 55 AMG '00 [350]
Mitsubishi Lancer Evolution VIII MR GSR '04 [354]
Nissan Skyline GT-R Vspec II '00 [355]
Pontiac GTO 5.7 Coupe '04 [355]
Renault Clio Sport V6 Phase 2 '03 [353]
A stock Lotus Elise 111R '04 comes with a little under 200HP, which makes it pretty quick and nimble to begin with. Add another ~150HP to that same chassis, coupled with the fact that it's already as light as a pencil shaving, and you've got a seriously quick competitor that given the right transmission settings could easily be geared to achieve 170mph.