A bright Sunny day, went for the Snowy Owl and Short-eared. I mounted my Kenko DGX MC7 2X TC between my 7D and my 300 f4L for a short while to have a try, and found it AF pretty fast on my 7D even using the centre focusing point only. Images are a bit soft, but looks OK after processing. Both are considerably big crops in spite of the 600mm focal length, and the in flight shot is actually a 100% crop with no re-size.