Check your exhaust pressure before the turbo. Might be an easy way to find a serious problem(Wrong turbine wheel for your housing, damaged wheel,open or defective wastegate,...)
Do you have an easy way to check exhaust backpressure before the turbo (o2 sensor bung or something)?
I can't think of anything else (at the moment) that sacarguy didn't mention or that you haven't tried.
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Ray
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