The USA in Decades of Darkness started out in a much stronger position than South Africa would have, more people, a much better climate than any of Africa save the far south, and much more territory readily accessible because of the whole natives-dying-like-flies-from-disease thing: and they still ended up only dominating the Americas while the Russian Federation is probably Number One Power by the end of the 20th century. You can probably get a Greater South Africa from the POD Stirling gives, but a hyperpower? You need an early historical divergence, I think. Here's one guy's effort, BTW...
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