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Assuming we want to be Marxist about this, isn't part of the definition of capitalism that it uses wage labor, and so slave-based economies are noncapitalist by definition?
^^^^^ CB: As far as being Marxist, Marx on this says, below, that slavery and colonialism were the chief momenta of the original or "primitive accumulation" of capitalism. In other places, he discusses the US slavery system as a main pillar of capitalism in the 18th Century. Capitalism = wage labor + specially oppressed labor.
http://www.marxists.org/archive/marx/works/1867-c1/ch31.htm
Karl Marx. Capital Volume One
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Chapter Thirty-One: Genesis of the Industrial Capitalist
The discovery of gold and silver in America, the extirpation, enslavement and entombment in mines of the aboriginal population, the beginning of the conquest and looting of the East Indies, the turning of Africa into a warren for the commercial hunting of black-skins, signalised the rosy dawn of the era of capitalist production. These idyllic proceedings are the chief momenta of primitive accumulation. On their heels treads the commercial war of the European nations, with the globe for a theatre. It begins with the revolt of the Netherlands from Spain, assumes giant dimensions in England’s Anti-Jacobin War, and is still going on in the opium wars against China, &c.
The different momenta of primitive accumulation distribute themselves now, more or less in chronological order, particularly over Spain, Portugal, Holland, France, and England. In England at the end of the 17th century, they arrive at a systematical combination, embracing the colonies, the national debt, the modern mode of taxation, and the protectionist system. These methods depend in part on brute force, e.g., the colonial system. But, they all employ the power of the State, the concentrated and organised force of society, to hasten, hot-house fashion, the process of transformation of the feudal mode of production into the capitalist mode, and to shorten the transition. Force is the midwife of every old society pregnant with a new one. It is itself an economic power.