Regarding high ISO hulk shots above. For me, it is not the best way to show quality of High ISO by these examples. With these lighting, you already get good exposure at 6400, that means, with the same lighting on higher ISO, All parts of the image is within the limit of ISO sensor bucket if you will.
Its a different scenario where in, you are in dark places, then even at 25600 ISO, u still get 1/2 stop underexpose image.
I am an amateur with less than a month experience on DSLR.
What I am saying is, for me, you can not gauge ISO performance in well lighted conditions, also,
the subject in your example is too big and maybe detail less in the first place.
I have not intelligently shoot my 6D yet, unlike my G12, I know its peak.
Hopefully soon, I will know its peak performance.. and see what peak is peak.