mechanically, it's prolly on the same level as a honda engine swap....maybe easier cuz the engine weighs alot less, however....wiring si the hardest motherfucking part. I was going to do an STI swap, but then I read a somewhat how to and you have to remove the entire dashboard and re wire just about all the under dash wiring...not really something I look forward to...there are quite a few sites you can go to....
www.rallispec.com if you have $$$ they will do the swap for you.
www.i-club.com do a search there on sti swaps and you'll get a bit of info
there are a few other sites, non I can recall though....
But for the level of difficult and money it would cost, you can build a 2.5L motor and have super gobs of torque...god knows I will....