As the ECU puts a specific amount of fuel in the chambers at specific time intervals in theory you could have a standard subaru unit remapped and have 1500 bhp churned out. Its the other mods that will give the most returns and the ECU should be remapped to ensure optimum running in conjunction with the mods.
You have a a good initial set of mods there and I would wager the ECU has already been remapped to suite.
Air in Exhaust out - done
next step would be more fuel in - uprated fuel pump and bigger injectors, high flow manifold - remap to suite
bigger bang equals more heat - not good sorted by bigger (possibly STI unit) or front mount intercooler
next could be bigger turbo but...
if you are going to run more boost you would need to look at ensuring the head can take it - thicker gasket to lower the compression ratio and uprate the bolts + oil pump
if you have the head apart you could look at stroker kits and forged pistons, lighter components reduce inertial mass and dont rob as much energy giving more bhp
you could transplant an EJ25 unit
taking weight out of the car and changing the power to weight ratio is a cheaper way of getting gains
what I would advise before starting on engine mods is to make sure the stuff you have is running at optimum, are the injectors and bores clean, fuel filter in good shape etc has the transmission fluid been changed recently with some quality lube have an engine flush completed then have decent oil in - all helps the mechanicals move with low friction and good lubricant flow. Know and be confident the point you are starting at before investing potentially large sums of money into bolt ons
hopefully food for thought
PS The feb issue of Jap Performance car has an article (p72) about a chap who races a classic 2000 model subaru on a budget, runs about 400bhp but mixes it with 800bhp nissans