The potential is there. There is a thread up in in mechanics about efficiencies of internal hub drives which includes a link to some good testing done on various hubs versus geared systems: http://bikeforums.net/showthread.php?t=363900

If the efficiency of the belt is at or close to that of a good chain and the hub efficiency is close as well, that would make one heck of a commuter/foul weather bike. If I am not racing, I would be willing to spot 2-3 % to get that. Heck, you can get more than that back by positioning & your choice of shirts.