check out my post regarding the S202.
same engine, from what I can find very minor changes. The biggest changes are ecu upgrade, oil cooler, and some silicone. They got 315hp.
The sti may not be that more powerful than the wrx but I am sure we will see that it is a better base for modification with more untapped potential and less risk (forged pistons, better cams, stronger gearbox, reinforced block). There is more to the STI engine than just a different turbo and intercooler (which incidentally many say is still too small).
probably if you just got the ecu mapped for 99RON (94OCT), it would produce significant power gains with no other modifications - the car has been detuned.....