After getting my 1D Mark III, I did what I have done with every new camera body since buying my first SLR in 1986 - I took it to the zoo. Melbourne Zoo has a reptile house; like most reptile houses it is dimly lit. This gave me a chance to try out the ISO 6400 capabilities of the Mark III.
As an exercise, I limited myself to just one lens for the entire visit - I took the 200mm f2.8L prime, partly because I recently read a critique that described it as amazingly sharp, and partly because I have always felt the need for a longish lens at the zoo. This lens is fast, sharp, and fairly light.
Anyway, here is a shot of a green tree snake, shot through glass at ISO 6400, f2.8, 1/500. Note that the same shot at ISO 1600 would have been 1/125, a little slow on a 200mm lens... The 100% crop shows that the image is surprisingly low noise (especially for ISO 6400!).
The uncropped image:
A 100% crop:
Not a scientific analysis - just thought you might be interested in the shot.
BTW: I had no focussing issues with the Mark III, but it was a cold cloudy day.