you could also consider using multiple physics scenes...( i think the dbPhySleepRigidBody commands are only for rigid bodies, how do they interact with vehicles ? i mean, i know the vehicle is made up of various rigid bodies, but would sleeping the rigid bodies affect the entire vehicle, or for instance, just its chassis ?)
that would increase the cpu load though and would need to be thought out, as to how it would be implemented..
for example, one physics simulation running with all of your "normal" physics interactions in it, and one with the vehicle and what it can interact with in it, that way, when you want your vehicle to leave the game(by being destroyed or whatever) you can just end that simulation and delete all its associated objects etc, leaving your "main" simulation unaffected.. ie. when you are "in" the vehicle, one sim would run, when u "leave" the vehicle, another would take over ... of course, if you are using more than one vehicle, that may not be feasible... and it also introduces a question that i am not sure of( having only just started using dark physics myself), is it possible to move physics objects from one simulation scene to another, and logistics wise, how badly would that affect the CPU do u think ?
If it ain't broke.... DONT FIX IT !!!