Ok, so I was mucking about with AF micro adjust this evening, and grabbed a couple of high ISO shots just because I could.
Now I know there are purists out there who will want to tear me apart for poor testing parameters, techniques etc, but these will do as real word examples.
Straight off camera, no adjustments other than converted to JPG from RAW through bridge and Photoshop, so probably some default settings applied by them.
These are full frame images no cropping at all, first 2 are a bit dark, but I really couldn't go much lower than 1/30th, all hand held.
No Noise reduction applied at all
7D ISO 3200 1/30, 120-400 Sigma @ 400mm f5.6
7D ISO 6400 1/30, 120-400 Sigma @ 400mm f5.6
7D ISO 12800 1/30, 120-400 Sigma @ 400mm f5.6