Thanks, turned it down another 3 turns. Getting in there gets easy with practice, I think it took me 15mins to get back to the nuts and adjust, then another 20 to close everything up again
Upshot is no difference, still peaking at 10-11 psi. I'm guessing that it's the wastegate valve not sealing properly or something of the sort.
Will likely just stick with the 10psi, as it's not bad on the highway...and it does have 400k miles on it, after all.