Stiff suspension is good for racing on concrete, because it's flat. Lowering the CG will help alot. I suggest moving your pushrods to the outer holes on the arms and taking out some or all of the preload. It will lower the truck significantly. If your using a tire with pins, you can cut off pins on the outer edge of the tire. It will help against rollovers too because you'll now have less side bite.

If you need to slow down a little to prevent a rollover, by all means, slow down! Don't flip over just because the jackass behind you doesn't know how to operate the brakes! Or slow down and go wide, letting the fast guys with tons of speed pass in the inside. It will end up working out better for the both of you.