Here are the results from Damien's Barts review: http://www.hardware.fr/medias/photos_news/00/29/IMG0029787.gif
At 800/1000, the GTX 460 is just 6% slower than the 470 (1920×1200 AA4X) and 1% slower than the 6870.
The GTX 560 could be up to 17% faster than that: (384/336)×(820/800) = 1.17. Let's call it 10% faster.
And here, the 6950 is just 7% faster than the 470.
So I'm expecting HD 6870 < GTX 470 < GTX 560 < HD 6950, all within a ~12% performance range, going by that last graph. I guess drivers could help Cayman, though.
The question than remains what are the prices ? If these are gtx460 replacements than they dont want it to cost close to 6950 and mainly gtx570. And if they will be priced near gtx460 than who will need a 3bilion transistor gtx570 ?