sounds like a wastegate leak since its only happening under pressure...

did you ever take apart the wastegate to change to spring? check and make sure the hex bolts going around the wastegate are tight and sealed, as well as the gasket inside hasn't shifted.

also just make sure everything has a proper gasket and is tight. most of the time since the wastegate is so easy to take on and off, ppl forget that the gasket gets worse everytime.
log mani's are pretty hard to get a good look at for this, but make sure there aren't any cracks or tiny holes... but then again you'd hear that all the time, doesn't hurt to check though.

when you start to hear your exhaust leak, does the car bog down any? lose boost?