as far as power is concerned the wrx has an extremely easy upgrade path. This is the path i've been following and will continue to follow ... tossing money into my car for the rest of my life. haha I'm 400 bones from step 5.

1) catless turboback exhaust
2) catless up-pipe
3) IC hoses or a new IC
4) gutted intake w/ high flow air filter (no CAI's)
5) engine management
The above combo will make your subie an Sti equiv.
6) upgraded turbo
7) upgraded injectors
8 ) upgraded fuel pump
9) upgraded fuel rail (protection from cylinder #3 issues)
** upgraded IC if not already purchased
10) retuned engine management
11) upgraded tranny/gearset
With all the above you'd be sittin at ~300awhp
12) upgraded exhaust manifold
13)upgraded bottom-end
14) blah blah blah