I'm certain alot of the 2.5 impreza stuff will apply to our application. But without tearing into the engine a cold air intake and complete exhaust is about all there is. You may be surprized with what a engine back(headers and everything) exhaust will do.
Once the air flow is improved, going in and out, you might consider a fuel reg. and slightly bigger injectors. More HP= more air in and exiting, and more fuel being burned efficiently.