'Shadow' style XTR is the lightest mech around.
More significantly, it's sprung normally for a super-light action with Rapidfire trigger pods. Shadow design tucks it away from rocks, too, and there's no second spring to let it bounce off the frame. It's proving more awkward to set up for consistent shifts than standard mechs, though.
This means the 'Low Normal' rigged Std mech still has its place, particularly with Dual Control shifters. Either way, Shimano's 'fluorine-coated link pin bushings', cold forged construction and the best jockey wheels around make XTR a super-tough unit.