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If you are running climb RPM, and running out of boost at 8500, I don't have any suggestions other than what you have already listed. To get cruise power above 16,000' or so, you generally need to be running 2500 RPM (for 65% power) or so, and you must make sure the wastegate is adjusted to *completely* close on the rear engine. It does not come this way when you buy it, and your mechanic might not be changing it because very few airplanes other than the P337 require the wastegate to completely close at high altitude.
But it sounds like you have a more severe problem. If you have no leaks, then perhaps you should send the turbo for overhaul to someplace like Main Turbo, and see if they can see a problem with it.
I am not an A&P, just a former owner who has experienced this problem, as have many at one time or another. Perhaps others on the site will have a better suggestion.
Kevin
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