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:
- Bugatti Veyron 16.4 [1213>1322 hp]
- Bugatti Chiron [1796>1851 hp]
- Fiat 500 F [73>??? hp]
- Ferrari F40 [736>??? hp]
- Mazda RX-7 Spirit R [499>586 hp]
- RE Amemiya RX-7 [512>537 hp]
- Nissan 300ZX [691>??? hp]
- Nissan Fairlady 240ZG [420>??? hp]
- Nissan GT-R R35 [933>??? hp]
- Nissan GT-R R35 Safety Car [945>972 hp]
- Nissan GT-R R35 Nismo [946>992 hp]
- Nissan Skyline R32 [717>??? hp]
- Nissan Skyline R33 [749>??? hp]
- Nissan Skyline R34 [785>??? hp]
- Subaru BRZ Drift Car [1195>1232 hp]
- Toyota Supra RZ [777>??? hp]
High-RPM S Supercharger:
- Chevrolet Corvette C3 Convertible [646>??? hp]
- Chevrolet Corvette C3 Stingray [778>??? hp]
- Ford GT40 Mark I [587>??? hp]
- Jaguar D-Type [394>??? hp]
- Mazda Roadster ND [269>??? hp]
- Mazda Roadster ND NR-A [283>??? hp]
- Pontiac Firebird Trans-Am [481>??? hp]
Bore Up S:
- Audi TT 3.2 Quattro [427>452 hp]
- BAC Mono [369>377 hp]
- Mercedes A 45 AMG [535>558 hp]
- Peugeot RCZ [335>355 hp]
- Toyota GR Supra '19 [704>732 hp]
- VW Scirocco R [488>516 hp]
Stroke Up S:
- DMC DeLorean S2 [315>331 hp]
- Honda NSX-R '02 [572>598 hp]
- Toyota S-FR Racing Concept [451>464 hp]
High Lift Camshaft S:
- Alpine A110 '17 [432>464 hp]
- Aston Martin DB11 [875>933 hp]
- Aston Martin V8 Vantage [688>735 hp]
- Audi TT 3.2 Quattro [427>??? hp]
- BMW M4 Safety Car [665>731 hp]
- Chevrolet Corvette C8 [897>964 hp]
- DMC DeLorean S2 [315>341 hp]
- Ferrari Enzo [860>914 hp]
- Honda Civic Type-R FK2 [462>492 hp]
- Honda Civic Type-R FK8 [508>540 hp]
- Honda NSX-R '02 [572>614 hp]
- Honda NSX '17 [852>893 hp]
- Jaguar F-Type R [755>808 hp]
- Lexus LC 500 [672>720 hp]
- Lexus RC F [665>713 hp]
- Maserati GranTurismo S [647>692 hp]
- Nissan 370Z [633>679 hp]
- Nissan Z Performance [757>796 hp]
- Porsche 911 GT3 RS (991) [686>727 hp]
- Porsche Cayman GT4 [532>567 hp]
- Renault Clio '15 [410>418 hp]
- Renault Megane RS/Safety Car [470>490 hp]
- Toyota Crown Athlete [597>639 hp]
- Toyota Crown Athlete Safety Car [579>619 hp]
- Toyota 86 GT/Subaru BRZ '15 [365>391 hp]
- Toyota 86 GT Limited/Subaru BRZ STI Sport [366>393 hp]
- Toyota 86 GRMN [397>??? hp]
- Toyota GR86/Subaru BRZ '21 [398>425 hp]
Of course, if any other cars are compatible with these upgrades, let me know and I'll edit the post.