Yeah, it turns out the boxer isn't the only thing in the engine bay. The transaxle sticks in there nearly a foot. The guy that is doing the Corvette/WRX swap showed how the LS1 fits in the WRX engine bay front-to-rear when the car is RWD only. The T56 doesn't intrude into the engine bay. This *might* give the car better F/R weight distribution.
One little thing that the 2.5RS has that should help is the hood vents (one over each cylinder bank) and a hoodscoop. That should help keep the engine cool.