Lighten the car! Take sandpaper or a dremel to the inside of the body and sand out a good 15% of the interior thickness. You'd be surprised how much plastic is in there. Will the car be easier to break? Sure, that's the price you pay for speed.
Reduce reciprocating mass on the wheels! Use a tiny flathead screwdriver to 'cut' 4 of the 8 spokes out of each wheel.
Painting your bit? Always prep the surface first; the paint will stick much better. Get some 800-1200 grit sandpaper and lightly sand the entire surface of the car. You'll find that your paint sticks much better.