I have watched a XMR in the Hatfield McCoys going up black trails, if it is mostly rock that machine can really start bouncing and jumping. He made it, but not the best on rocky trails with steep grades. They tend to really hookup on the rock, if they don't go forward you bounce up.
It really depends on what type of hill your climbing.
I've climbed straight vertical banks 4 feet high with the right burp of the throttle
Because of the longer wheel base it makes it a lot easier to climb steep hills but as mentioned the silverbacks tend to buck a bit and the bounce of these tires can do amazing manouvers if you do it right, however the thing to be weary of is the higher center of gravity on the XMR. She will tip easy and roll and unless your clipping along at a decent pace I wouldn't try and turn around on a steep incline as I'm sure we have all seen the video of the guy who rolled his XMR down the mountain on YouTube.
Earlier this year I made a climb up a mountain side only to realize it couldn't be climbed with out the proper weather conditions, had I tried to turn around on that incline I'd of wrote my quad off. Luckily there were trees to winch my self slowly down and unfortunately others weren't as lucky as I found snorkel parts scattered in the bush on my way down the bank.
Had mine on the outlaw trails in West Virginia about 6 times now. Its taken me up everything I have wanted to cliimb but those silverbacks are for mud, not the hills. Alot of nervous moments. Not to mention if you putmuch of a sideload on them they will pop off the rim. Done that 3 times now. Just bought a set of MSA bead locks and will be investing in some trail tires soon.