Yea, dont tell me its impossible because i know it's not.
Have performance modifications availible at transfenders. (eg. Weight reduction kit, Engine performance kit, tires)
All you'd have to do is edit the vehicle's row in the handling.cfg file. Its easy. There, you can edit the mass, and aerodynamics of a vehicle.
if someone selects weight reduction, their vehicle's mass gets lowered. Easy.
The hard part is making it vehicle specific. Modifying the mass on that particular car, instead of every car of the same model currently on the server.