hmm... it's hard to say what the future might bring My MR2 turbo I went a bit overboard with and ending up polybushing the whole car, changing the turbo, re-doing the brakes, literally rebuilt most of if and in the end, there always was something that needed doing. To be honest, the mods I was thinking of were (not in order): aftermarket turbo-back exhaust (not loud if possible), BBK (unnecessary in the STI), possibly some uprated / slightly lowered springs, perhaps uprated ARB's, but in terms of power not sure. I always prefer a car that turns/brakes good rather then accelerates stupidly fast but then struggles to stop!
Any mods have to be done keeping in mind that this will be my daily car (approx 140 miles/week, most of the time less than that)