Anyway just assuming they are right the 6870 is sub 50% behind the 580gtx, the 6970 has a circa 70% effective increase in shaders over barts, do the math.
one 4d shader should provide about the performance of one 5d shader, so shader for shader they are about 25% faster. Meaning 1536 shaders is equivilent to 1920 shaders, but with BARTS front end, that meanst 1120 up to 1920 = 71% increase in shader power, roughly.
A 1536 shader Barts would be gaining around 40% shader power, I'd frankly expect a 1536 Barts to beat a 480gtx comftably and be not far off a 580gtx anyway, potentially beating a 570gtx(with a increase in memory clock speeds to bump bandwidth with more shaders). Cayman could be potentially adding another 25% from shader performance with better shaders, more 4d shader clusters meaning better performance in worst case scenario's.
But other limitations might mean that much shader power on a 256bit bus, and similar internal bandwidth means it might not scale up with shader power perfectly. Theres also the possibility it could scale better with more tweaks to the memory controller and front end. I think the 1536 should be close to a 580gtx, potentially 10-15% faster anyway, if its got 1920 shaders, and the 4d shaders do give a 25% performance boost, it could utterly destroy the 580gtx.
