Shot this today with the 5D MkIII from the office and my 400 5.6 + 1.4x TC (Kenko). Despite the high contrast tree in the background, the AF locked on to the bag in an instant and tracked it right along. I have a whole series of shots. I neglected to go to AI Focus, so the focus wandered just the tiniest bit in later images. I also tracked moving people and some cars, and they all stayed nicely in focus.
I'm thinking now the 5D MkIII with its accurate f8 AF may be fine for me. Right now it's still the least expensive option of the 3 I mentioned earlier.
