As an XC owner and my ride buddy the proud owner of an 800GS, and another friend with 1200GS (i have ridden) I can offer you my 2 cents worth.
If you intend sticking to fire-trails only plus the road all 3 will do it easily. The big 1200 eats road miles up easier than its smaller brother, but the XC will easily stay with it. Just revving more.
The "little" 800's will do over 200KPH so there is no issue being slow on the road.
The triple engine is a peach.
The 800GS is rough over 4200RPM.
The XC will cruise at any non legal speed easily with no vibrations.In my opinion and my 800GS mate as well the 800GS gets vibey after 110KPH and is fatiguing. He rarely goes over 110KPH (70MPH) for that reason.
Its generally not a problem as we are adventure riders and like lots of off road mixed in with twisty mountain roads. The tiger is faster on the road.
The less weight and 21" front wheel of the 800's make a huge difference when you end up in technical off road riding. My 800GS friend and myself like you have a strong MX / Enduro background and love the balance of the 800s.
I also like riding fast in bitumen twisty roads and that's why I got the XC over the 800GS.
I can easily keep up off road in technical trails but the 800GS has a more off road feel to it and carries its weight a bit lower as well.
As a rider with a strong dirt background you WILL venture off road as like us the fire trails got boring and these bikes will surprise you as long as you understand they are not like EXC KTM's or in my case WR450F's.
So for me its the 800's, or a KTM990ADV. Its the 21" front wheel you will appreciate in the long run.