I think (hope) the low rail was from the loose water separator on the fass, I've only had that code come up twice from a cold start and never while driving after it's cleared. Also never had any issues holding rail pressure before granted that was on stock injectors and briefly on 40 overs.
Will see if the code comes back but im pretty sure it has more to do with the RPM it's shifting into 5th gear, if you're in the throttle hard it holds 4th longer and shifts into 5th at a higher RPM keeping the turbo from falling off after the shift. Also runs fine if I have it in manual and shift to 5th later or manually kick it down to 4th, if I try to roll into the throttle at highway speed to pass someone it just dumps smoke unless I manually shift to 4th.