the STi catback is 2.5", not bad, but not great. get a good downpipe, thats where the power will be gained.
the catback is more or less for the sound/style. the STi probably has the best sound deadening.. the BB turboback (BBexhaust.com) is really nice, and i almost got it before i got a full Invidia TBE for a silly price. anyway, the BB has a classy lookin twin tip design that would look sleeper and sound wicked.