Hey Waz, nice rig matey. I to have a GU 4.2TD with the same mods you have , I put Mickey Tompson ATZ's on just yesterday, $1680 for 5, I put 285/75/16 & noticed quite a difference in power loss, not that it was that powerful before, it also threw the speedo out by about 6klms at 100 k's.
Did you feel much in the way off power loss?

And if anyone knows I have seen a thread somewhere that says you can buy different gear drives for the speedo from Nissan ,to fix the error, meaning if I have a 16 tooth gear fitted, I could buy a 17 tooth gear to fix the error.

Cheers, Rick