I know your looking for suggestions 'based on experience' but I don't have an 800gs. But I have done quite a few miles on TKC's on my 1200gs.
Last September My other half and myself covered 2700 miles around the Alps, she on a 650gs (single) and me on the 1200. I rode on TKC's fitted new for the ride. Jill had Pirellis fitted, though I can't remember which ones except that they were road tyres (the bike was sold on shortly after our return).
Both bike where fully loaded with all the kit, plus those extras necessary for ones comfort. The tent weighed 10Kg

and both bikes had large alli panniers and 42ltr top boxes.
Upon our return there was no significant ware to either set of tyres. Both bikes performed very well with the exception of the TKC's when riding on the cobbled hairpins of some of the passes. they gave the impression of riding on flat tyres, but with no loss of grip.
The 1200 has now done 9000 miles on the same set of TKC's and there is still 3mm left on the rear. I have been getting 14000 front and 11000 from the rear with the TKC's and comfortably touch my toes on the corners. As for the 650 I'm still in contact with the current owner and have not heard mention of new tyres as yet, and he is commuting two up about 40 miles daily.
With our experiences I would have thought that you would actually need only two sets of tyres to get you there and back. I would be confident of getting about 7000 miles from the rear TKC whilst fully loaded.
Tourances where mentioned by Sijohnstone. Though as he mentioned not the EXP's as they are too soft. If your going to be riding on tar and gravel these will do you well with excellent mileage and road handling. They do come in 90/90/21 to fit the older R100gs's and also the HP2, so therefore should fit the 800 nicely.
Though it's not directly 800gs experience, I hope this gives you some idea as to what you can expect on your travels.
Good luck with it all.
Val.