The Trick
I have stumbled upon a trick that I don't think very many people know about. You can remove the turbo/supercharger from some cars. In order to remove the forced induction from certain cars, they have to be cars that come with either a supercharger or a turbo (obviously), and they need to have the option for an alternate form of forced induction. For example, the 1998 Toyota Supra comes with Twin Turbo's, but in the upgrade shop you can swap in a single turbo. The ZR1 Corvette has a supercharger but you can swap in a turbo etc. I will list all the cars I have found that this trick works with below, but first I will explain what to do.
In the Upgrade Shop, go to "Conversion" on the far right.
Go into "Aspiration Conversion" and buy whatever is available in there (i.e. Turbo, Supercharger).
Go to "Engine and Power" on the very left and buy the highest level of turbo/supercharger available (usually stage 3).
Now simply sell that part, and your car will now be naturally aspirated and have much lower Hp.
This trick is very useful for getting awesome cars into much lower classes, and also makes these cars eligible for any events (In game or Online) that restrict the use of turbos or SC's.
This trick will work with the cars below, and with any other car that can accept the motors from these cars. (i.e. Many Chevy's have a ZR1 swap, so if you don't want 638hp in that car, you can make it 494hp to be a little more manageable in, for example, an old muscle car).
Personally I enjoy many of these cars with lower power (i.e. Koenigsegg CCX, '83 Porsche 911 etc).
Here is a list of known cars that it works with.
BMW 1M '11 - B487 335hp to C414 253hp
Chevrolet C6 ZR1 - S633 638hp to A587 494hp
Ford GT - A578 550hp to A522 412hp
GMC Syclone - C358 280hp to E262 201hp
GMC Typhoon - C358 280hp to E262 201hp
Jaguar XKR-S - A524 420hp to B452 308hp
Koenigsegg CCX - R3712 806hp to S644 497hp
Lancia 037 - C362 202hp to E266 155hp
Mazda RX-7 FD - B441 261hp to C372 205hp
Nissan Fairlady Z '94 - C379 300hp to D301 236hp
Panoz Esperante - A527 420hp to B484 338hp
Saleen S281 - B447 365hp to C383 289hp
Saleen S5S - S623 650hp to A569 487hp
Saturn ION - C392 205hp to D294 153hp
Toyota MR2 '89 - E272 145hp to F192 113hp
Toyota Supra '92 - D290 210hp to F200 161hp
Toyota Supra '98 - B461 320hp to C386 242hp
Porsche 911 Turbo '83 - 296hp to 230hp
Hope it helps.