Well if you're sure it's not an internal issue it sounds like a wastegate issue. At the same time, if you feel the car is the same as before, well I don't get what the issue is. Also if it feels like it's making the same power I doubt you have a dead turbo. If anything maybe your wastegate took a crap. and is opening early and bleeding off exhaust.
What is the target boost on the map, and what is the taper supposed to be? My tune I'm pushing 20psi peak tapering to 15 by redline, and it tapers pretty quick.