Search the site or Google for "DIY Boost Leak Tester". Very easy to make and VERY helpful. People who think their system is perfect are often surprised with the results and make more power after addressing the issues.
I can't remember if it's been addressed, but check for error codes, do a compression test and test oil and coolant for contamination.
EDIT: Oh, and the last thing is to check the mechanical and ignition timing. A cam on which the belt has jumped a tooth can cause similar symptoms.
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