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A proper pressure test on the intake tract is worth doing — “looking and listening” can miss small leaks that only open up under boost. Also double-check the wastegate seating surface and make sure it isn’t sticking or leaking around the valve. I’ve seen similar creep when the wastegate flow path was restricted by carbon build-up or a mismatch with the headers. Since he already tried different springs and even removed the boost control, I’d lean toward a mechanical flow issue with the gate itself or the way it’s plumbed.
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