maybe out already in japan?
they don't have the ej25 in japan. there are very high taxes on cars with engines over 2.0L. the JDM STi has the ej20.
i imagine, though don't know for sure, that the block will be very similar. however, all the internals will be different. i know the STi ej20 has 80% different parts then the standard WRX unit. the STi 2.5L recieved all the same treatment as the 2.OL. sodium filled exhaust valves, variable valve timing, forged pistons, oil squirters on those pistons, etc. the block my have been changed to incorportate the oil squirters.
as for fitment in the GC chassis, it will go in. the question is will it bolt right in or, more likely IMO, will it require some loving, especially in the crossmember area like the WRX motor, to fit.
stock WRX runs 14.5psi. STi specs haven't been releashed but the US spec EVO runs 19psi.