I don't know if it will help with your decision, but I have a Thorn Audax which I use in it's most rugged setup, due to me weighing 15 1/2 stone and the local roads being fairly bumpy and potholed. I use the fattest tyres it will take which are 28mm for normal tyres and 32mm for Panaracers. Also I use a suspension seatpost to absorb some of the shock and improve comfort. I use this for lightweight camping touring too, putting 10lbs on the front and up to 20lbs on the back. I also ride offroad on it sometimes in summer. Lowering tyre pressure helps too, about 85psi rear and 70psi front is good for me.
My other bike is an MTB converted tourer, so is not a direct comparison with the XTC, a few lbs. heavier and using 1.75" tyres. It's a bit slower than my Audax but feels more robust and stable.
Hope that is of some help.