We went Munich to Cape Town on Metzeler Tourances on an F650GS (single) and an R100GS. We didn't do any serious off roading and found that knobbies would only have been useful in northern Sudan and northern Kenya. I think our total distance from Munich was about 23,000 km and the Tourances made that no problem. I don't believe that knobbies (even TKC80s) would make it that far and then you're forced to find tires somewhere in the middle of the trip.
'84 R100RT 141,000 km (Dad's!)
'89 R100GS 250,000 km (and ready for another continent)
'07 R1200GS Adventure 100,000 km (just finished Circumnavigating Asia)