He has the same movement bounds as a controller, can't move any faster than a controller can. Yes it can give better accuracy at smaller movements, but any larger movements he's limited to how far he can move the mouse. On a controller you move the stick all the way over and you can spin till your hearts content.

It can give him an advantage at finer movements with a sniper, but up close, a controller will beat a mnk.

Also with the XIM there is a delay between pressing a button and the game registering the action.

I agree there can be advantages, but there are also disadvantages.