Cantilever brakes have such a range of setups that virtually any brake can be setup with as much mechanical advantage as any other. In fact, nearly any brake can be setup with too much mechanical advantage. I don't think Shorty's have any more or less potential mechanical advantage than the 720's. Being lower profile they do have a smaller range within which they can be setup for high mechanical advantage.
Check out this link for a visual representation of what I'm saying:
http://www.circleacycles.com/cantilevers/