580 is going for about $550
5970 is falling $500-600 so they're comparable segment.
5870 is $300
6870 is $250
If you slot Cayman at $400 it has no price competitor and shouldn't compete with 5870.
6870, however, does. It's almost the same performance for less.
That makes an assumption Cayman is starting at $399. It is also possible that Cayman starts at $299 because:
6850 $179
6870 $239
6950 $299
6970 $399
6990 $599
Fits quite nicely as a price range. It also fits in with the pricing of evergreen and actually exceeds the launch prices for both the 5850 and 5870 by $40 and ?? forgot respectively.
Now if they scale price/performance equally in proportion to Cayman as compared to Barts, Cayman Pro will be 20-25% faster and Cayman XT will be another 20-30% faster again.