Seriously, (40/28)^2=200% is just a rough estimate. There are also many other factors to consider, especially Amdahl's Law - increasing the shader count without significant improvement in the architecture doesn't necessarily bring much performance increase.
AMD is likely to continue on the sweet-spot strategy - maintain a great performance/watt ratio, then gain halo effect from dual-gpu on a single card.
nVidia is likely to fight for the king of single gpu and gain halo effect in such way.
Er.... OK?
