Since no one else is responding....
I'm in the same boat...wanting to get back to one bike, but more off-roading. Have a 1150GS and KLR, sold the KLR yesterday. That was a good combination having a KLR & 1150GS but the 1150GS was just getting too heavy for me for any off-roading and KLR was not a good solution (comfort-wise) for long over the road travels (ie: planned trip from Atlanta to Portland, OR).
Bottomline: I was fortunate to test ride an F800 for 8 hours and in my opinion it's a GREAT bike. The F800 will handle 80+ mph on freeways all day long with no sweat, but heavy luggage (ie: Jesse's), long saddle days would be more comfortable on the 1150GS. My 1150 is not even straining to deal with 80-90 mph loaded down. Never any wind buffeting on the 1150GS but would except to be getting blown around by big 18 wheelers on the F800, but never encountered that on a 100+ mile freeway ride.
On the negative side my 1150GS is set up exactly the way I need it and the F800 would probably take $3,500+ in farkeling to get it set up like my the 1150GS. On the positive side, my 1150GS gets 35 to 37 mpg whereas the F800 would probably provide 55 mpg.
Major changes for me would be:
1) Major seat upgrade
2) Mid height windscreen
My opinion is you'll definately give up some comfort factor on the F8 but should expect that with a smaller F800GS.
And remember: life is like a roll of toilet paper. The closer it gets to the end, the faster it goes.