Those off-topic interjected opinions to simply pass judgement on others will almost always cause confusion and waste time. We can get back on topic, with the 2.5" level and that offsite, I'd still look at the 305/55/20 if you want widest and tallest you can fit with the possibility of a little rubbing with the +1 offset. Although, you might be able to squeeze something taller but narrower in there. All depends on that rim offset you choose. That's what really will determine the size of tire you can run. Check stores or online retailers that specialize in wheel tire combos and give them a call. You'd be surprised on what they can determine over the phone or email. Also, take a look at the 2014+ Wheel and Tire Setup topic here: http://www.gm-trucks.com/forums/topic/161228-post-your-2014-wheeltire-set-up/?st=0 Might give you good look at what to expect with that offset and size. I know there's some guys on there running that rim with A/Ts. Just a question of their lift/level after that.
FYI, The most popular A/T tires I found being ran on here when I was researching them seem to be: BF Goodrich All-Terrain KO2, Toyo A/T II, or Nitto Terra Grapplers G2s.
Hope all that helps. Hopefully someone with knowledge on or running that wheel offset with the a 2.5" level can throw in some valuable info.