Sticking my 2p in here....
In my experience standard qrs are pushed to the limit with decent discs. Even the 185mm hope c2 on my commuter is able to pull the left side of the hub down a fraction of a mm on a very hard stop. It's enough to get the pads to drag until I either stop and retighten the wheel or hit a large bump to reseat the wheel in the dropout. I need to find a very tight clamping skewer, I may even try the old chrome shimano ones as they used to be good.
The other issue I've found is one that I've also experienced on old cars converted to discs that haven't been setup properly and it's one that stems from the master cylinder end of the hydraulic circuit.
I'm not sure if XT levers have it, but hopes have a pushrod adjustment. If its too tight and the master cylinder piston can't return all the way then the system appears to gradually tighten up. On cars it can result in a wheel completely locking up on its own as the system gets hotter progressively, but on bikes there isn't the heat levels to do that so you just get a bit of drag. I've found that just the tiniest bit of clearance between the piston and pushrod seems to be about right.
Like I say I don't know if this adjustment is available on the XT system, but it's worth mentioning.
My money's on the hubs moving about in the dropouts though.