Well... I tried it with the Maserati Gran Turismo S '08 and it didn't show any more power than usual. I actually did the change to brand new car that only had wheels/ aero from gtauto. The I installed the Veyron turbo/ controller and then I went back into the game and did oil change (where you usually lock-in and see new figures with turbo swap) and only had 448hp/ 369ft-lb at 500PP. With ECU, intake, filter, cat, header, and exhaust (all sports) it's only 522hp/ 406ft-lb at 522PP. The car does weigh 1780kg and only makes power to about 8,000rpm, so if I got a car that revs higher it would probably take advantage of the Veyron's turbo setup.
I'm gonna try another car now. Well, that or see if I can increase the rpms that the engine makes power in.