ok here is what is happening just trying to get some opinions.

2000 maxima se 31k miles

was running a zex kit. w/walbaro high flow fp ,b&m FPR 2 step colder plugs.

problems

car back fires
if he gives to much gas the car dies.
when he accelerates slowly his top speed is 20mph
fuel pressure gauge is reading 5-10 psi. after the dealer ship got doen with it. he said it was reading about 20psi before.
low idle (on the verge of stalling)

he was in for warranty work and supposably they replaced 2 o2 sensors,fuel pump, and air flow meter.

dealer called nissan and they came sdown and they found the nitrous line. and void all warranty. we took everything else out of the car.

they want $2200 to fix
fuel pump
fpr
fuel dampner


this is just about all i have to go on. but if the dealer says these things needs replacing i can do these things. what im worried about is what if it was not these things what then?? anyone know of anything that can cause all this. need some ideas.
if there is more info needed i will ask. i know its not that much to go on but any ideas would help.

thanks