I find it hard to believe it will be better then the current 5D with so many more MP.Even though it has been pointed out on here time and time again that more pixels does not mean more noise...
And time and time again thats been shown to be incorrect. Given sensors of the same generation, and at high ISO, the larget pixels of a lower mp camera will have less noise.
It might be interested to compare a D800 with the Canon D1 x at ISO 51800. But wait, the D800 can't do native ISO above 6400, (if the spec is correct). Why, because the noise would totally destroy the image.
Yes the deal is rather that at say ISO 400-3200 the difference between high res cameras and high ISO cameras is not very big in practical use in real prints. However on higher ISOs the high res cameras start to drop off in performance and can't do the really high ISOs at all. So if you need to work in the ISO 6400 - ISO 25600 range then the advantage is obvious with the ISO kings, up to ISO3200 less so, on base ISO no advantage at all. So it depends on how you want to use your camera.
An auto ISO mode up to ISO12800 would surely be useful at times, for example indoor sports... but as said, if I just have to pick one property, my shooting style needs the resolution.