You can talk about over 5970 performance all you want, AMD place it behind the 5970 in performance, looks like 580 speed to me.
Oh boo hoo, I said 5970 instead of 5870, impossible to tell what with suggesting 1920 shaders is a 20% increase.
As for the 5970, its 40% ahead of a 580gtx in some games, 50-60% ahead in Call of Juarez, it can be both slower than a 5970 and faster than a 580gtx, either way, where in my post did I mention 580gtx performance, and since when has "it only been about performance at this end of the market", it never has been, and never will be, its about price.
If a 6970 is 10% slower than a 580gtx, and costs 30% less, it wins, if its 10% faster, and costs 10% less, it wins, if its 50% slower, and costs 70% less, it wins, etc, etc, etc.
If the 6970 is 10% slower and gives them pretty much the same profit per card selling at £300, as a 580gtx makes sold at £450, and has more availability, then it will sell 10 times as many cards purely because a heck of a lot more people can afford a £300 card than a £450 card. Thats how life works, the 4870 wasn't very close to a 280gtx in performance, but it was less than half the price, and outsold the 280gtx with absolute ease.
The 5870 isn't faster than the 480gtx, yet it outsold it with absolute ease, value, and price are the key factors, performance only matters if it increases value or is completely unavailable elsewhere, there are lots of combinations that beat a 580gtx, and for a lot cheaper.