are you running a panhard bar at the back? that's the next step i'm looking at along with a locker and some real stiff shocks. lowering it makes a pretty big difference along with some camber, castor and toe out to stop the understeer. if you go some real stiff spring try and match them with some good stiff shock absorbers, otherwise it'll just bounce everywhere like most lowered cars.
so what's it doing at the moment? just starting to run wide and understeer? wallowing a bit? have you locked the diff? the more specifics you give the more likely people will be able to help you
also semi slicks make a huge difference, and if you're running 13's they're cheap as chips.