if we're talking the new STi vs the new Evo X - Evo all the way. Early testing by several top US tuners such as AMS, Vishnu, and Buschur are all reporting the same things - huge gains from typical bolt ons. Intake, test pipe and exhaust (plus a few other minor tweaks) are getting the car in the 350whp range (dynojet), which is 11 sec territory. Vishnu has gotten 45% gains from just a catback, test pipe and tune - 50% with a race gas blend. I have not heard of any car that picks up 100hp with just bolt ons - let alone ones that can be bolted on in under an hour total. Early reports are that the car is exceptionally easy to work on as well, making the new Evo a tuner's dream come true.

But, like I said, these are early reports. I have not heard as much information on the STi yet.