Sounds to me like air as well. Here's something to try. It's also free (apart from fuel costs).
Take the car for a gentle drive without too many hills in such a way that you recognise the lack of performance without using too much throttle or engine load. It's vital that you don't stress the car right now and still be able to tell if the problem exists.
Return to the starting point and remove the top plastic cover of the engine. You'll find a vac hose that runs across from the vacuum pump on the vehicle's right hand side, this runs over and connects to the boost control solenoid. Remove it from the BCS. Attach it to the top of the actuator.
Now take the car for another drive and drive it along the same route with the same minimalist power. Now is when it's important to NOT back off suddenly. The turbocharger - if it's working properly - is making full boost at all times. A sudden drop in RPM will cause an overboost and this may result in a hose popping off the intercooler or intake manifold (I know, I've popped mine from the intake manifold when trying this).
Put the hoses back to the way they were (vac hose from over engine to BCS, reinstall other hose to actuator).
Was there a difference? Was it remarkable? If so, your BCS is shot. They don't simply fail all the time. They become erratic, they become annoying when they won't misbehave while the mechanic is looking and as soon as he's looking at something else it plays up again. The BCS is like a evil garden gnome.
If the BCS has failed, you have 4 options.
1) Replace it with a genuine item. Yes, absolutely, it WILL fail again because this component was manufactured for Nissan by Evil Gnome Manufacturing Co. However, your car will produce exactly the right amount of boost that it was designed to do, it won't overboost or underboost and your fuel economy will be as it was.
2) Directly connect the vac hose from the pump to the turbo. Dangerous, cheap and potentially catastrophic results. It's include here because it's a possible option.
3) Replace the BCS with a Tillix valve (tillix.com.au). This is what I've done. It's easy to install, but you have to be careful when setting it up - don't let it overboost. Your average boost should be around 16psi (which is lower than the BCS allows for, so the engine is a little down on power) and it will peak at around 21psi. I had mine set to 18.5psi average and it peaked at 24psi and popped the hose off my intake manifold. The fact that I was towing our caravan (who can say "Bathurst 1000"??) along the Putty Rd at the time didn't make it very comfortable, especially seeing the masses of black smoke covering the caravan. Yuck. Reconnect, back the valve off a smidge and it has been fine ever since (yes, we've been back to Bathurst again since then for Challenge Bathurst).
4) There's an alternative to the Tillix valve and I can't remember what it is. I hope someone who does know and remember will come along and let us know what it is.
As above, a bluetooth OBD adapter and an Android phone with the Torque app is very valuable, but since you have an iPhone you may need a different app (and as Boogie points out, the WiFi OBD adapter).
