My last bike was XT/XTR with only the rear derailleur being the XTR part. My current bike is full XTR. Honestly, I can't tell a difference in performance. If any of my XTR stuff breaks, I'll replace it with XT. I'm not a weight wienie, so the few less grams means nothing to me.
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger