Dealing with Geelong Nissan Spare parts is like pi*#ing in the wind. Werribee Nissan have always been REALLY helpful for me and I've been there quite a few times. EG: Needed CA18DET head bolts and rod bolts... Geelong said they couldn't do it at all without a VIN number - no matter how much other info i had. Werribee had them in for me the very next day, at a good price too.
Are you able to put in another battery - even if it's a standard one so you can see how long IT lasts. If you've changed nothing else in the meen time, it will prove that it is the Optima Battery.
What about a shortened Borg Warner diff? There is some really quick VL's out there with a heap of tourque (and they weigh a lot more than the Datto). They don't 'suck' as much power as a 9inch either.
I have been told by a very good source that the strongest standard holden diff produced was the VS supercharged V6 diff. Apparently stronger than any V8, N/A V6 or Straight 6 live axle diff.
Hey PIGDOG... I just went out and meassured mine. I have a T25G - The outside diameter of the rigid return line is 16mm and the same for the sump. Hope it helps...