Just about any 3 inch exhaust will do as long as the downpipe is either divorced or bellmouth (basically just don't get the hks downpipe). I've heard pulleys or (not and) a lightened fly-wheel help a little. But yeah, then get a protune and you should be set for awhile. If you want more after that, I'd go with a bigger intercooler/turbo and supporting mods. I've known people that run meth and they usually have problems with it (nothing serious, just having to replace pumps constantly). Personally I'd avoid meth or nitrous unless you are looking to create an absolute monster.

Oh yeah, and like everyone else has said, don't get an intake or bov. They are a complete waste on a mildly tuned sti.

PS, Definitely check out WRXAtlanta, IWSTi, or Nasioc. This is about the last place I'd look for advice on Subarus.