The width of the blade (braking path) is very important. I think Deores had a wider rotor than the XTs. If the pads don't fully contact the rotor, you're more likely to suffer squealing, stuttering, uneven pad wear, etc, etc.
Get a DT-Swiss adapter and run a good set of 6-bolt rotors.
A forum community dedicated to Mountain Bike owners and enthusiasts. Come join the discussion about bike parts, components, deals, performance, modifications, classifieds, trails, troubleshooting, maintenance, and more!