Subaru superceded the parts used for their boost control restrictor pills sometime for the '05 MY. What this means, is that it's VERY difficult for turbochargers on said vehicles to exceed 15-17 PSI on a factory equipped IHI. Stock WRX's will achieve even less. If you've found that your in this category. You have 3 options. (If you are trying to sustain over 20PSI at redline, just go to option C) A: Leave it alone and tune the car for what you can get out of it. B: If you have an adjustable waste gate arm, you can preload it based on conversations with your tuner. C: Install a GM boost control solenoid (part number 1997152). A search on NASIOC [Click here for one of the posts] can yield quite a few threads on the proper installation of this part. Again, it's very important that you do this with the guidance of your tuner. If not, you WILL suffer from boost spiking, or worse. word.
In the below thread I was asking about the GM boost solenoid, in post number 6 veck mentioned a solenoid spring trick using the stock solenoid.. Could this also be an option for you, and does anyone want to explain what the "trick" is? http://www.wrxatlanta.com/forums/showthread.php?t=7598&highlight=boost+solenoid