That'll be a great trip. The XT is made for it too. There will be a lot personal choices on kit / spares too.

I'd zip tie your spare clutch cable to the original for a pit-stop swap. As for the throttle cables, they're not as easy to swap over out in the field so, for me, I'd put the new ones on before you go. Same with clutch plates, if you doubt the originals will last 15k.

Perhaps have a look at a new rectifier too. These have been known to be a gremlin waiting ro wake up.

It's probably more about carrying less weight whilst future proofing the bike.

I'd (this is just me now) practice removing both tyres with tools that you plans to carry. That way you get to find out what you actually need and what you forgot about. (New tubes, +patches+ some sort of lube).Punctures are a fair possibility and it's not fair that that experience should leave you in a flap on the plains of Mongolia.

__________________
I have a lot of growing up to do. I realized that the other day inside my fort.