Originally Posted by DriftKing
No it isnt. The STi motor is a closed deck and has Dohc (if you can call them take) as opposed to the SOHC the 2.5rs has. The Sti motor has an 8.0:1 compression while the 2.5rs has a 10.0:1. It def cant handle boost as well as the Sti motor can. You'd be lucky to get 9psi from that block, stock of course. It would be a much wiser investment (cheaper too) to swap in the ej20 motor. Also, the transmission on the 2.5rs is actually from a 1.8 Liberty, it cant handle big horsepower either.
You're right about the tranny, definitely different. Plus you get the neato DCCD business if you swap it. I was always under the impression that Subaru only had two different 4-cylinder block designs, one for the 2.5 and one for the 2.0 engine, and they just swapped heads depending on which car it was.
I'm pretty sure though that the swap for the STi motor is straight bolt in, so if you got the cash it definitely wouldn't be hard to turn an RS into a pretty fast car.