I was in this same position with my stage 2 AP. Check all the vacuum lines to and form the wastegate, make sure there is the restrictor pill between the "T" in the vacuum line and the compressor. if all those parts are intact and not leaking, it very well may be the wastegate solenoid. I have a friend with the same year WRX as mine, we traded wastegate solenoids and it turned out to be the problem. I found a used one on the for sale forums for $50 shipped ($150 new from the dealer).