My new-to-me XTR has 2500 miles on it. I'm happy with the performance and the economy, but I'm just wondering whether it will continue to loosen up and improve, or whether it's already as good as it's going to get?

I should think it's pretty much done, of course parts will continue to wear and loosen up, not to the extent the first couple of hundred would have done ,
Don't think modern stuff really beds in to much these days,

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K+N filter, snorkel removed, homemade kev mod (with his blessing), co raised,
Pipeworx cans, r1 led back light, under tray removed, small indicators, handguards, cnc short levers,

Just rag the ass of it.