I got a 2.5 x 18" from John Titman Racing here in Oz. He also supplied a set of heavy gauge stainless spokes cut to size, and the rim drilled accordingly too. Bit $xxcy, but simple enough to lace the stock hub, and much stronger. My initial reasoning for doing this was primarily that the big trip to the Simmo and Kimberly happening soon would be accompanying bikes with 18" back wheels only and this would economise on spares, and 18s are a bit easier to come by in outback Oz if required. The raising of the rear 1/2" was good too for ground clearance.
I fitted and balanced a Mitas 130/18 E07 and got 7,000 kms before I scrapped it after a cut carcass and puncture. Maybe 2,000 kms left. It was mated with a balanced MT21 front. I've swapped them for another E07 and 606 front.
The E07 seems to get traction OK most antwhere, but is not real good for side grip in slop. Good compromise though, like the DR. The MT21 just worked too, and would fit one again. Its good that the MT21s are unidirectional too, as they scallop up a bit and need to be turned around. the 606 is directional. we shall see...
Balancing is good, and recommended.