So people say you can't use ISO 25600. Last night I shot this photo at that ISO and a shutter of 6 seconds just to test location before I dropped the ISO to 6400 for the real shots. When I checked the shot out on the computer, I really liked it and the noise wasn't so much that I couldn't clean it up a bit in LR. Granted night skies are a little easier to deal with and the photo isn't perfect, but this is about as much noise as I would get with my T2i at ISO 1600!
The 6D is really impressive with high ISO. I have quite a few perfectly usable shots at ISO 25,600 and don't hesitate to use it. I just sold a fine art print last week that I shot at 25,600.
Here's another from the 6D-vintage Helios 44-2 combo.
A Fungus Amongst Us by Thousand Word Images by Dustin Abbott, on Flickr