It would be a very marginal gain, but as a 50/50 road and mtb rider I do prefer the lighter/stiffer feel of a road shoe/pedal when riding hard on the road. There's a reason why mtb shoes/pedals have evolved slightly differently from road shoes/pedals. Both are ultimately optimised for their intended purpose. Now whether it's worth $250 is another matter. If you are expecting to go faster then I think you would be disappointed. But if you needed new shoes anyway then I would do it. FWIW I currently ride SPD-L on road and flats for mtb.