In regards to XT or SLX... I think for most riders, if SLX and XT derailleurs are tuned and maintained, differences are negligible. Have been for me, anyway. But... where I've personally noticed a difference is durability. My experience is SLX isn't as tough and has worn out sooner over time. In a couple of extreme examples, I've had an SLX derailleur wear out (get sloppy/loose beyond repair) in two seasons, versus I have a 2003 XC bike that still has it's original XT derailleur going strong with 1,000's of miles on it. Again, these are two extreme examples.
Another personal durability observation is when derailleurs get banged up (like, on rocks, etc.). Never really had an XT suffer shifting issues when getting bumped. Have had SLX's require some in field bending. Not a scientifically controlled test though.... every bump is different.
In regards to XTR... had a couple of XTR derailleurs over the years. For whatever reason, both needed too much tuning to keep shifting tight. Like, in every few rides. Not sure why. I know it's probably not common, but because of my experience, I've just stuck to XT.
Lastly though, like Flushing Shadow, my OCD (or CDO, in order like it should be) tendencies would push me to all XT (or whichever level) for consistency.
