If you made a real life 450MPH tweak-beast and started throwing it about hard, i've got a funny feeling you'd be beefing things up to take the strains of pulling 5g's+
ATM it's mainly the fact that the cars are virtually standard except for engine and transmission tweaks, that lets the program down a little, so i'm trying to stick with more realistic tweaks for now. 2Ltr XFR's and a 1380cc (roadish spec+tyres) UFR to simulate the average Fast mini etc.
It's amazing to me how little you have to do to a car to change it into something compeatly different