The RTR rusty is fairly slow out of the box. I think using a 'typical' sport's pack, you're looking at about 18mph.
However, these things can get up and boogy. They're very heavy so first thing to do is lighten the chassis and other parts. Get a nice ESC and a stock/modified motor. The 20-turn stinger motor is... well.... pissweak.
Keep in mind the stock rusty does not come with bearings so there's a bit of speed to be had with that simple mod. Go for the RPM upgraded bearing carriers as the standard 5x8 size will smash after one hard landing.
The good news is that these trucks are unbreakable.
Check out www.ultimatetraxxas.com
for some ideas. Ive done mine up to "Phase III specs" for very little cost, a lot of fun and it's *nearly* competetive on the track.