I shoot with the same combination (7D + 100-400) and haven't had the focus issues you mention, but I do see that all my shots are very grainy, even at ISO100. Except in extremely bright light on a very bright surface, I *always* have grain in my images. It's not so much of an issue for printing through 13x19, but is noticeable on screen and for sites like this can cause issues (not that this is my only problem getting HQ
photos on here). I've always attributed this to the 7D sensor, not the lens. Other IQ
issues are my technique, I'm sure.
This sample 100% crop is not a HQ
shot, but I don't have many samples on my work PC
. Either way, you can see the extreme (IMO) grain present in the shadow and sky.
Shot at ISO100, 1/3200, f2.8 at 200mm