So at this point, I'd say it's pretty common knowledge that almost every road car in the game has unlockable Titanium Conrods and Pistons, and almost every FR and 4WD car has an unlockable Carbon Propeller Shaft. There are also plenty of cars with unlockable Stage 5 Weight Reduction (although this just reduces the weight by the same amount as Stage 3 would on other cars), but what about the other 5 power upgrades? Below, I've listed every car that is known to be compatible with these special upgrades, from both this thread and elsewhere. If the HP difference from installing the upgrade is known, I'll add that too.
Ultra-High RPM Turbo:
- Nissan Skyline R32
- Nissan Skyline R33
- Nissan Skyline R34
- Nissan GT-R R35 [933>972 hp]
- Nissan GT-R R35 Nismo [946>992 hp]
- Nissan GT-R R35 Safety Car [933>972 hp]
- Toyota Supra RZ
- Nissan 300ZX
- Nissan Fairlady 240ZG
- Mazda RX-7 Spirit R [499>586 hp]
- RE Amemiya RX-7
- Falken BRZ Drift Car [1195>1232 hp]
- Ferrari F40
- Bugatti Veyron 16.4 [1213>1322 hp]
- Fiat 500 F (yes, I know)
High-RPM S Supercharger:
- Chevrolet Corvette C3 Stingray
- Chevrolet Corvette C3 Convertible
- Pontiac Firebird Trans-Am
- Mazda Roadster ND
Bore Up S:
- VW Scirocco R [488>516 hp]
- Audi TT 3.2 Quattro [439>470 hp]
- Mercedes A 45 AMG
- Toyota GR Supra RZ '19
- BAC Mono [369>377 hp]
Stroke Up S:
- Honda NSX-R '02 [590>622 hp]
- DMC DeLorean S2
- Toyota S-FR Racing Concept [439>456 hp]
High-Lift Camshaft S:
- Toyota Crown Athlete [615>657 hp]
- Nissan Z Performance
- DMC DeLorean S2
- Aston Martin DB11
- Porsche 911 GT3 RS (991) [686>727 hp]
- Ferrari Enzo
Of course, if any other cars are compatible with these upgrades, let me know and I'll edit the post.