Make sure its not your dump valve, I have had a weak standard one that leaked at high boost. Easy to check just block it off with something (I bolted a bit of plastic under the flange to test).

 

i had the same problem with my build

was the dump valve , as said just block it off

another strange one i have seen ( not on a subaru ) a bolt had dropped down the upipe during the install when the car was switched on the bolt lodged it self in the turbo causing a blockage

i think the question you need to answer is how is it making its current boost

im sure Neil will sort it out