Mine has always got good fuel consumption, even when it had the stock ecu and running in to the 10's at full throttle.
However I did notice at WOT around 3000-3500rpm it would hover around the low 13's. I was told it was normal as the Nissans run a little leaner in the mid to help fuel consumption.
On the dyno afr's were spot on 14.7:1 and would hang in there until 2psi of boost was being made. That was with the stock ecu and the rb20det.
It has never really backfired or popped. Only if I were to ever so slightly have my foot on the throttle.
A few things I would be looking at.
Fuel injectors, AFR's, O2, ECU's Water Temp sensor (one for the dash, one for the ecu), cam timing, compression.