I updated the firmware of my 5D3 and took it out for a test drive with my 500f4 II + 2X III. Normally I would shoot this combo on a tripod, but today a Red Tailed Hawk flew directly towards me before I got the tripod set up, so I dropped the tripod and took the shots. I have to say it is pretty tough, most of the images were soft or out of focus, but this one looks Ok to me.
5D3
500f4 II + 2X III
1/1600
F10
ISO-1600
Evaluative Metering of the Blue sky, + 1 1/3 EV
Manual Mode
Hand held
Cropped to about 8 Mpixels
I brightened the under wing and tail as Shane suggested a little bit here using Curves. Looks OK to me.
Regarding the AF performance, I'd say it is not bad at all. It picked up the hawk immediately. Tracking, however, is not so good, but I'm sure it has to do with me trying to steady the rig and not trained the AF focus point right on the hawk. There were a couple of shots that the hawk fully occupied the frame, but was out of focus. I should have nailed those by pumping the focus perhaps, but I didn't do that.
I would probably have had more keepers with a 1.4X in the same situation, but it is always fun to push the envelope to see where things fall apart.