I was talking to a guy here at work who's on the waiting list to buy a new WRX. He's really into Subaru (especially their Rally team). Anyway, he said that the major differences between the US WRX and the WRX STi is going to be the size of the turbo. It also has a more restrictive exhaust and the AWD/Differential system will be a viscous one and not electronic. Otherwise, I believe engine internals are up to the job of more serious boost and it's ready (compression is 8:1) -- all you need is a new turbo and a boost controller. Plus I've heard you can put on a factory STi boost controller that will let you dial in up to 30 lbs. of boost -- and it's all backed by your factory warranty ;)