nissan engines tend to run richer than hondas, on stock everything, so increasing the FP is not gonna do any good at that point...
increasing the FP will make you run a bit richer, so if your AFR is say 14.0 at WOT and by increasing FP from 3 bar to 4 bar will allow you to run some 13.5 AFR then you should see an HP increment, but if you running rich and make it richer thats gonna do no good