just buy the BRZ, add all the suspension, exhaust, intake, and manifold goodies to the regular one for probably HALF the cost that the BRZ STI is going to cost...
THEN, call up a local engine tuning shop (one that does machine work) and have them bore and stroke the flat four out to bring up the displacement, add bigger injectors, custome tune, and a better clutch, and you should have a decent car and maybe get close to 300hp at the crank...
It may cost a grand or so more in the end, but you will have a car that has the power it should, AND it will still be an NA motor!