I've not always been sure that the 6970 will be faster than the 580, I've been hopeful but not sure. But seeing Overclockers expect to sell at £400 I'd have to say faster. From the rumours yields are fine, even with 2GB of ram as standard AMD wouldn't charge that much unless (very worse case scenario) it was within 5% of the 580.

Basically the 580 is as near perfect as Fermi can get on 40nm, it's a strong card and AMD wouldn't charge equivalently unless they had a winner. Also neither company has let their PR dogs loose AMD has stayed silent that in my books means there's no damage control to be done.

The maths you've done hoping to figure Cayman out is very interesting, but we don't even know the real number of shaders. All we know is that Cayman is about as big as AMD is comfortable making, and that AMD could beat a 480 with a core slightly bigger than Cypress just using the optimisations found in Barts.

At £400 they are on price parity with both the 580 and the 5970 (which easily costs more to make) in the UK, which only means one thing, it wins.