Look. Roughly 3% of the value of US GDP is imported from countries where the real wage level is less than half that of the United States. In most cases the cost advantage that third-world have over first-world firms is relatively small. Even if the prices of such goods are only 2/3 of what they should be because of imperialism, that's still only a 1.5% boost to U.S. GDP--and a boost that is distributed proportionally to income across the U.S. population.
If you want bigger numbers, you have to look at the Spanish crown or the Spanish aristocracy of the sixteenth and seventeenth century, at the London merchants of the eighteenth century, or at the southern planters of the nineteenth century.
Brad DeLong