I would recommend the following mods:
magnus makes a sheetmetal intake manifold for the subaru WRX and STI. That will be money well spent. The plenum is much much larger in volume. The runners are shorter and wider.
After that I would try ordering a ported set of heads. Your heads have a lip on one edge of the intake valve to promote turbulence and better fuel mixing. This helps with torque and emissions. If you are not happy with the manifold try this out.
This is all power that can be had with out increasing the boost and minimizing the added strain on the motor. Frankly, I believe in building up hte base airpump before killing it with a bigger turbo.
