Check out AHG for driving courses (they're out near the international terminal). Simple mods to begin with which are also supporting mods for later are:
Highflowing filter
Exhaust (done)
Bigger intercooler (depending on power levels you're chasing you can go front mount or R34 side mount)
New fuel pump (stock one will be getting old, something along the lines of an Bosch 040/044)
Manual boost controller (Turbotech boost T, wind it up a bit and see how it goes - i.e. leaning out, hitting R&R etc)
IMO it's at this point in time you should start to address the braking/suspension issues. Good luck and happy modding!
P.S. My little RB20 has pwned 2x R33's now