I’m assuming the air filter is good, and all the turbo/intercooler pipes have been checked for leakage. Also the intercooler itself.
A fairly small leak anywhere in the pressurised intake system would cause a lot of the symptoms you describe, and only be evident when the engine comes under sustained high load.
Check your turbo boost gauge when you are pulling up a bank. If it doesn’t go right up to the top of the scale then something is amiss.