subie engineers have already tweaked the 2.5 as far as possible, yet not trading off long term service.
can't stroke a 2.5 (much) can't bore it, aftermarket cams help a little but cost the stock motor's fat midrange torque in return for a little extra up high (bad deal for dirt lovers). the stock 2.5 is a really sweet dirt/snow/mud setup because it's got better low/midrange torque than a 2.0 WRX all day long. it's not so much the absolute numbers, it's how the 2.5 delivers torque--right NOW not waiting for turbo to spool.
ECU can help a bit. we ran a Link standalone ECU on our 2.5RS rally car (expensive box, HUGE wiring challenge too) . with intake, headers, exhaust, the Link and a dyno tune, we got up to about what a stock WRX puts to the wheels. lots of $$$ for the gains but the car went like holy nell just the same!
stick to amping up performance with suspension, tires, maybe gearing like a 4.44 box and diffs if you are ready to shell out $$$.