it's a bit heavier
rapid rise reverses the direction of the shifting, so your levers will do up on a down and down on an up.
it's a bit slower, not that most people would notice it, due to the angle and distance from the top pulley to the cassette. closer and more parallel to the ground gives smoother, faster and crisper shifts.
other than that, there's no real downside.
oh, I think I've heard that the latest generation of XTR rear derailers do not match up to road cable pull ratio. The front derailer never matched up between road and mtb, but barcons in friction mode could fix that.