Check the vent tube on the tial wastegate. If its like mine its not connected to anything and venting to atmosphere. I experienced an overboost issue once and turns out there was a bunch of dirt clogging the vent fitting. Cleaned the fitting and re-directed more upwards and all is good. Probably should check it again now that I think of it.
Okay I just re-read and saw that you guys checked the wastegate. I got nothing. lol. Hope you get it sorted and please update. I run a similar setup.