the best you could hope for is higher quality, lighter faster wheels but by the point this would start making any truly noticeable difference you would be out of the price range of it making sense to put said wheels on a goddamn MTB. This is the kind of the kind of conversion you do when you have spare 700c's out the yin yang, multiple brakesets to play with(optional), and the modest technical know how to make it all happen. the sensible thing to do if you want a MTB that is roadworthy is swap the knobbies for slicks. mountain bikes worked fine for decades with 26 inch wheels opposed to 29's I'm sure an inch or so makes even less difference on the road.