Two stops of ISO? I don't think that's theoretically possible. 1/2 stop of ISO is a big difference.
The difference between the original 5D and the 5D MK III is less than 1 stop. The original 5D is the lowest scoring Canon FF sensor (as far as high ISO is concerned) that DXO has measured.
Even the 1D X is a hair less than 1 stop better than the original 5D.
I believe what I meant was if the mkiii is usable at iso 6400, then the mkiv would be usable at 25600.