Don't leave us hanging Jack, what were the issues you found?
I am running a 350 shot of nitrous with a single 255 fuel pump and voltage booster. The car is about 850 rwhp. I log fuel pressure because I know I am at the high end of the output. I bring the fuel into the rear of an external fuel rail on the passenger side. From the front of that rail I go to the back of the passenger side rail and the two fuel solenoids.
The car runs a consistent a/f on both sides when running NA. That is at 525 hp. The first time at the strip (after the install of the two O2's) with nitrous I found the drivers side was one point leaner. I flip flopped the O2's, but, it stayed the same, the drivers side was a point lean. The light went on, the drivers side fuel rail is the farthest from the pump and it is after the the passenger side and the two nitrous fuel solenoids, really simple, it is a plumbing problem resulting in a pressure drop at the end of the system.
The most important issue is that I routed it similar to the factory and that is not totally right. The only difference is that my plumbing is external. My guess is that most cars with external rails are plumbed the same way and this is not the right way for high hp applications. The correct way would be to transition to a wye and then go into the back of each fuel rail.