Here are my gains after meth. Now granted this is on a 2.0L WRX, not an STi, but this is just to give you an idea of the gains availible with a good tuner.
Setup is a VF22, UTEC, MRT TMIC, HKS downpipe with user added wastegate dump tube, TurboXS uppipe, Perrin Short ram, stromung catless exhaust, Snow performance kit using 50/50 meth/distilled water mix. I want to dump the UTEC, and go ECUTEK or Protune. I am tired of UTEC. That or I will go HYDRA.
Anyway. The thinner lines are my pump gas map. The thicker/darker lines are the water meth tune. Tuned at 27PSI, but only run about 22-23 on the steet. Your gains should be more profound once you change the turbo, and get another tune. You should gain quite a bit with the stock turbo though.
Maybe it was a little less than 27psi, but you get the idea.