This is really nice idea.. Driving around with gravity 40 (assuming that regular gravity is 800), jumping from a ramp.. Or gravity 1000 with cars to avoid collapsing in 90degree turnings.. Ah, the wonderful world.
Whine whine whine, all you do is whine. Human race has that kind of a character that it always seeks for the worst parts of other people, never seeking the good parts of anything other.
Please check that your Vice City has not been modified. Try to reinstall Vice City. Or just read http://forum.mtavc.com/viewtopic.php?t=11098 to find solutions to some problems.
Pretty weird idea.. Only point I see is stunting, and I believe that modding that to MTA:SA core would be useless.. But could that be done with blue by some third-person programmer?