This.
Its been stated since day one Southern Islands was a stop gap between Evergreen and Northern Islands. It's been said that it would take the core from evergreen and match it with the uncore of northern islands. So based on that alone, I think most of the performance improvements will be in other areas other than just raw power. Logically it makes sense.
Maybe this analogy - kinda like the same CPU core getting a better chipset with more features later on its life.
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