I think this one needs to be listened to. There are a few possibilities for it. Just for starters, disconnect the NPS and get that right out of the way. To diagnose the problem with the least amount of expense, we have to remove all of the possible causes that don't cost anything to do.
Pop the bonnet and have someone rev the engine to 2500rpm. If the air hose between the turbo and the intercooler (or intercooler and intake manifold) collapses, you're sucking more air than the car's capable of getting so it's either one of these two:
1) Turbo not spooling. This could be turbo failure, it could be a sticky wastegate, the actuator may not be working (faulty actuator, faulty vacuum hose).
2) You could have a great big fat bird in your snorkel (don't laugh, it's happened before). Check to see that the grill of the snorkel hasn't been damaged. It could be a clogged air filter or something wrong in the baffle, it's worth holding the air filter in place with the top off and cranking the engine over to listen for the sound it makes and see if it revs any more freely.
If the air is okay and you're sure the turbo is actually spinning (and hasn't failed - open it and spin the impeller - cold engine!! - it should spin smoothly and the shaft shouldn't wobble) then it could be fuel-related (it could also be timing-related, that's next).
3) Check the hoses around the fuel filter and the primer bulb for cracking or hardening. Look for hose clamps that are too tight - they'll pinch the hose and allow air in.
Is there a lot of black smoke when you put your foot down and it "limps"? If so, it is definitely getting the fuel, it's probably 1 or 2 or it's timing. If not,
4) If you're not confident that you have a lot of fuel flow (which is hard to tell unless you put a clear section of hose on the outlet of the fuel filter to the pump) then maybe the fuel pickup is partially blocked. It'll let a certain amount of fuel flow through, but when a high demand is placed on it, the restriction is too much. This would also happen if the fuel hose had been clamped, like it was jammed above the tank when the tank was remounted. Have you had a long-range tank fitted, or any other fuel tank work recently?
Failing a fuel problem, the only other thing it could be is timing, and there are two main influences for this.
5) CAS. Without knowing the proper position of the crankshaft, the ECU can't inject the fuel at the right time. A misalignment of the CAS puts the timing out of whack. It's something I'd ask Nissan to check.
6) Timing chain. Had to say it - the YD25 chains aren't perfect. Not a huge number of them break, but enough cause trouble to warrant us thinking about it. Best way to tell is to grab a $25 mechanic's stethoscope from SCA and have a listen to the housing at the front of the motor.
I think there's enough there to start with. There are other possibilities but you end up with a sore head reading this much guff, so let us know how it goes.
Personally I'd be checking the fuel pickup after the visual checks in the engine bay, because it sounds like (a) you've got a minor clamping issue at the front of the fuel tank allowing air to be drawn in under extreme suction and b) something's blocking the pickup inside the tank, which causes the extreme suction AND loss of power - but this guess is based on NOT knowing whether you're blowing heaps of smoke or not.
