Some people have been able to hit 500 with a single 255, but it always sucks to leave the tuner at 450 because you "ran out of fuel".
I didn't know you could setup twin in-tanks on fox's. I know with modular cars you can buy hats to hold twin pumps, but I'm curious now myself.
Edit: I found a fuel pump calculator, I can't vouch for how accurate it is.
At 10psi and 500rwhp with stock fuel pressure. It rated a demand of 322lph.