This has been bugging me for a while. The Ultegra has no visible wear. The chain has no measurable stretch. The Deore is silent even when dirty and not oiled. The Ultegra is much noisier when freshly cleaned and oiled. After a minor crash which resulted in a bent derailleur hanger on the Deore equipped bike, I straightened it without a proper tool (obviously not perfect) and it's still quiet. No crashes with the Ultegra bike. The difference is so big it's not even funny. I know you're not supposed to cross chain but I find front shifting so inconvenient (large paddle throw, slow shifting compared to rear, you know what I mean). I'm looking for the hard facts, what is it that Shimano did that it's improved so much? My guess is 11 speed (more lateral flexibility in the chain) and different derailleur pulley wheels design but I'd like some expert opinions.
I am not sure this is a fair comparison. 6600 ultegra is now three generations old (6700, 6800 and now R8000 come after).
I rode 6600 for many years, rear shifts are still among the fastest out there. But as you noticed cross-chaining was not great.
I moved to 6800 last year. Shifting speed was compatible (I still feel that a well maintained 6600 is faster, but we are splitting hairs), but cross chaining is much better.
When 6600 came out in early 2000s 11-25 was a monster road cassette for climbing. My current road bike has a 12-32 and I would not go back to a smaller cassette unless I was racing cat 1/2 again.
By comparison XT M8000 became available on bikes just a couple years ago so it represents a much newer group set. A lot has changed in intervening years, and Shimano has tweaked their offerings to match market demand/ current trends.