I really think the best investment besides beefing up the suspension would be to take some autocross driving classes. A decent suspension setup is only as good as the driver. You will be able to make the most out of what you have when you learn proper techniques instead of relying on self-taught bad habits.

New tires and a thicker rear sway bar (with proper reinforcement) will be a good start. That will be very noticable even with stock suspension. Then, start saving for a good set of coilovers.

I am unsure how they determine classes, but I am sure that all that suspension work would put you into a higher class with more competetive drivers that know their setups better and can use them more strategically in getting the best times. Someone correct me if I am wrong.