I re-read the post, didn't realize AJP noted your wastegate duty cycle was 90% and you were making 5psi.....
IIRC, with the wastegate on the charge pipe, if the springs fail for some reason, it can get stuck open.
Ideally, if you have someone to check with you, you could hold the brakes down hard and put a small load on the engine. This might provide you with enough boost leak noise to assist in pin pointing.
I would think with that much pressure loss you are looking at either the wastegate failure, or one of the charge pipes came loose from its coupler. It may not be visually obvious as when the car is off there is no pressure, so you may need to use the Braille method to feel for loose clamps or pipes on the back side of the engine.
ALSO, they do sell boost leak kits, you plug up the charge pipe and it has a schrader vale on one side... you add pressure with a pump until you hear the leak and locate it.
http://turboboostleaktesters.comKeep us posted!!