It will be interesting to see how this plays out once the tank class hits the game. I'm hoping that one of the advantages of the tank is that it's harder to force aside or shunt off the track, and conversely, I hope it can just ram it's way through lighter cars (at the expense of speed and acceleration).
Speed should still play a part though - a lighter car with more momentum should still have a decent effect on any other car if they are moving slower relative to said car - everything subject to natural laws of velocity and angular momentum vs friction/gravity of course. More of that fun physx stuff I guess. Saying that, maybe a mechanic would be nice where you can break to a full stop and take reduced impact force if you hold down your hand-break. Risk being you just blow his ass off the track with a missile