You should find that the z32 air flow meters will give you a larger tuning resolution that the stock ones,
but there really is no way that the Z32 AFM's can give you a power increase on their own.
The stock AFM's will run out of resolution/max out and have no more Voltage range to give at approx 300rwkw, i have had this exactly on my own car, then the Z32's are installed to give tuning resolution past this mark.
The power gain you have shown is most likely because of a diferent day on the dyno, some other modification you performed, or most likely it is running a bit leaner on boost or have more ignition timing.
Not having a go at you, just letting you and others know, that AFM's, just like upgrading fuel pumps and injectors will not give any power increase, but they will support more power.
The only other way a pair of Z32's will give more power on their own would be if the stock 65mm AFM size was restricting the motor and the 80mm size allowed the engine to breath better. The Nismo AFM's are 65mm though, with the tuning resolution of the Z32 AFM, so really, unless we are looking at 550+rwkw, i cant see that being the case .