IMO it has to have improved high ISO noise over the 5D III to even open a discussion of buying one. Otherwise I see no worthy benefit over the 5Ds R for my purposes. The frame rate isn't fast enough to make it interesting either.

The 5Ds R had a new mirror vibration control system, I am sure this is part of the improvement with the AF system. I can tell you the AF system of the 5Ds R has been on par with my 1D IV and is very accurate. But whether those improvements translate from the slow fps 5Ds R to the 5D IV remain to be seen.

As for focus speed the 5Ds R is not as fast as the 1D series, I doubt the 5D IV will be any faster. I think that is a power and battery issue.