Dedicated "off road rig" is apples to oranges compared to a poser cramming in the biggest tires that fit.I've met quite a few people running 39s or 40s. Sort of opposite of what @Big Blue said, they are dedicated off-road rigs. In addition to glass/carbon, you generally have to shave the firewall and headlights, which basically ruins the headlights. You'll definitely want to change gearing as well. All-in-all, not terribly difficult, but pretty invasive. In the Gen3 world, the ideal would be ordered with 35s, but running 39s.
Experience obviously from Gen1/Gen2, but no reason to think it wouldn't be the same for Gen3.
And I still have never seen anyone need anything bigger than 35" for blue/black trails. At least not in the northeast.