Why a fuel cut defender? If it was cutting fuel then it would be a aggresive misfire not just a loss of power. Come to think of it, I dont even think the GTR's have any form of fuel cut for "overboosting" anyway... certainly not at the boost levels you are talking about anyway.
You have said loses power, not ignition cut/misfire so its not an ignition problem.
The o2 sensor is only used on light throttle cruising, at WOT it isnt being used.
My money is on the fact you have upgraded to turbos that can flow a substantial more amount of air than the factory ones, and the ECU isnt liking it one bit.
You really should be running an aftermarket ECU for the modifications you have if you want it to last (or at least find someone that can remap the standard one) or else you will end up with a shit engine.