I've been playing with the physics, unfortunately it is not possible to precisely set the cars' characteristics.
What matters: downforce and engine power, both for acceleration and top speed. If I set it to the correct acceleration, its top speed is way too high; if I set it to the proper top speed, its acceleration is crappy.
Maybe drag has something to do with it.
Currently it is 0.35 downforce and 0.39 drag; its top speed is about right without performance packs, but its acceleration - even if it says 2.5s - is over 3 or 4s.
Here are already existing Veyron physics:
http://forum.turboduck.net/showthread.php?t=18642
http://forum.turboduck.net/showthread.php?t=17857