welllp, Boxer can claim "what the Subies should be makin" all he wants, but tuning gains on a Scooby N/A motor are going to be incremental at best. don't expect your 1.8 with an intake and exhaust and a lil ecu twiddle to give you WRX type power. the Subie factory puts the motors pretty much to the limits of what they can handle long term straight from the assembly line.
for example: we built a 2.5 N/A for the Last Ditch RS rally car, which turned out to be a very successful racer--one of the fastest non-turbo Scoobs on the North American rally scene. despite major tweaks like cams, intake, full race exhaust (aka straight pipes) , injectors, and a $3000 Link race ECU, it barely came up to the output of a stock WRX. It'd have been cheaper and far easier to have just swapped in the turbo motor.