It's an interesting topic but sys is arguing something quite a bit different and far more insane.
Whether slavery resulted in a more efficient economy isn't the point. Southern landowners had already expended significant capital to acquire their labor force (which drove their entire business and most of the southern economy). Freeing slaves would be the equivalent of taking the machinery out of a manufacturing plant and saying "hey, the machines are just as valuable as they were before, you just have to start renting them now." The fact that slavery may not have been that profitable overall means the economic impact of emancipation would have been that more significant given the sunk costs. You need profit to grow.