Its not the physx, i think its desync. If you play single player I doubt you will have these problems.
when you desync and pop forward a few feet your traveling at a much greater speed during that desync. So instead of hitting a wall at 50, you hit the wall at over 300mph.
Thats what it feels like to me. Vehicles also randomly explode, that feels like desync also, maybe clients catching up and throwing vehicles about as they desync.
imo