As people have said, it's not what the rack can carry, but other effects on the bike. You don't want the rear end of your frame to fracture half way through Africa when you are riding on rough roads, or to damage your shock, or weave at motorway speeds, etc. It's not just a separate bolt on subframe which you can bolt on a new one, it would need braced and welding. I'd say 20kg will be fine on smooth roads, but for your trip limit that to 10 or 15kg. (it doesn't sound much, but 15kg is the weight limit for a suitcase on a lot of cheap airlines nowadays, it's not an insignificant amount of luggage!)
I also sling a pair of soft panners over the seat, hanging over the plastic side panels, in front of the exhaust and behind my knees. This is a necessity with soft panniers on the XT because of the exhaust, but what it does mean as well is you are carrying the weight a bit lower and a bit more centrally.
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