[robtherobber]

Not contradicting the second part, but I want to emphasise that they are different things. Slavery (and capitalism) can be extremely inefficient and simultaneously wildly profitable.

[n4r9]

Surely it's meaningless to compare the efficiency of slavery vs other systems, since your set of resources is completely different.

[tgv]

You could if you look at e.g. the crop yield (ceteris paribus). I don't know why you would, because what sane conclusion could you draw from it?