While it won't completely fix vehicles in A3, I'd recommend trying out Red Pheonix's "Refined Vehicles." He's gone in and messed with gearing and torque for most of the vehicles and is continuing his work. I have a feeling he'll get to the Karts eventually, if he can. The addon just changes the default values, so you don't need to place new units in missions.
Last night I was playing a self-made mission where I had a MRAP that I was driving all around PR Afghan Village and it's so much nicer than the BIS values. Initial acceleration is as you'd expect (sluggish) for an armored car, but once up to speed, it actually shifts when you want it. It even has compression breaking, so when you come off the gas, it starts to slow, but not in the same linear fashion BIS values do.