hey paul,
sorry if it came across as a member attack that wasn't my intention your 1st post was vague so i didn't think much from it.
from my experience with 3 skylines with similar mods they are able to get 400-500km out of tank with a problem, so leads me to think that is something other than the ecu ...
you mention putting that on a dyno, i think that is the best opinion for $40-$50 powerun you will your afr, which can explains alot.