OK a few more updates:
Removed ALDA, no change still 5 psi boost
Removed CAT was pretty clogged up, thought that was it, nope spins faster to 5psi but wont go over.
I had removed the crossover pipe to check for obstructions, found none, noticed a Turbo mounting bolt (right at the wastegate) was all the way backed out and covered in soot. Ah-ha!, tightened and sealed but nope, still 5psi.
Disconnected the actuator pressure line, still 5psi
Closed boost controller completely, but no effect on the boost still 5psi.

What's left to check? I have no visible leaks on the turbo compressor housing, or in the plumbing. I hear hissing when under load so I assume that the wastegate is dumping prematurely regardless of the signal it gets. Is that possible/reasonable? Am I missing something else? Can the turbo leak internally with no external signs? Is this a wastegate spring issue? I would have thought that the boost controller would override the wastegate, but it's not, so can the turbo simply dump boost due to a weak wastegate spring?

What is the range of motion for the wastegate? Mine moves about 3/4 in from one side to another when applying compressed air, is it possibly stuck open? I would imagine the diaphragm can only move <1"

TIA

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)