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Finally fixed!
I finally got my car back and it is working and looking like new. This would have been no big deal had the replacement parts been readily available but, unfortunately, they weren't. The piezoneumatic waste gate actuators failed resulting in low boost pressure. The dealer did extensive tests to prove there were no vacuum leaks. After diagnostic proceedure, the waste gates went from occasionally sticking open momentarily to doing that on every acceleration. After waiting 2.5 weeks for the parts, replaced them and declared victory.
BMW has seen this before for the x6 35i (but apparently not other N54 engine models) and has a service bulletin out. Apparently replacement parts are still in short supply so don't bring your car in for the symptoms (rare "engine malfunction reduced power" that clears after a re-start) before verifying availability.
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