Do you mean in the twisties or on the highway?
I love the X-Challenge but I'm not aware of any thumper that can comfortably keep up with the big twins on long stretches of highway. Different tools for different jobs. Of course, anything rougher than a well maintained Forest Service trail and I'd happily trade a KTM950 or 1200GS for the X-Challenge.
Well, both. I didn't say anything about "comfortable"
Seriously though, it does a surprisingly good job cruising at 70 - 75mph down the highway. I came from an F650GS (same motor) and before I bought that bike that was my major concern. The X does have more issues with wind blast from big trucks but I've gotten used to it.
Two up though, while it has been done, probably isn't that comfortable though. I'm sure the 800GS would be much better at that.
As far as the 800GS being better off road than the 1200GS, I think the 800GS having a 21-inch front wheel is a big help especially in the rougher stuff (ruts and such). Both are heavy and both require some good skills to ride serious off-road.