from cobbs website....how to check if it is a mechanical problem with the boost control system
"How can I test for this with my AccessPORT?
You can flash a Realtime Economy Mode calibration which turns your boost control system off so it will run the minimum mechanical boost pressure, which should be 5-7psi for the 2007 WRX and 8-12 psi for the 2007 STi. Using the boost measurement feature of your new AccessPORT, you can see if you exceed the mechanical wastegate spring pressure. If you operate your vehicle while under wide open throttle conditions with the Economy Mode calibration and you notice your boost levels greatly exceed these mechanical pressure levels then you will have to mechanically revise your boost control system, see below."
flash the econo map, get on it, and see if you're showing boost running past 7psi.
here is the entire page...says it's typically a problem if running a catless exhaust on stage 2, but may be relevant....
CobbTuning.com - Service Bulletin: 07 WRX/STi Overboosting with Catless Exhaust