I'm trying to recall but the number that jumps out to me for base 18" rears is something like 22lbs. Knowing the 275 RE11 I've been running are 31lbs that should be roughly 54lbs, so really I'm looking at an extra 3lbs with the new wheels and 345's. I can live with that. Good thing the new rims are lighter though to compensate otherwise it would have been real easy for the weight to be much greater than it's going to be.
Thanks for making me contemplate these things more specifically than I otherwise may have.
The 3% difference in diameter remains the piece I'm curious about how it will affect the end result now.