When I purchased my Canon 800mm 5.6 L Lens I knew it would not work on auto focus with either a 1.4 or 2x Extender. Alas, I purchased a Canon 1DX and after downloading the latest firmware upgrade, I was pleased to discover that now with the 1.4 extender autofocus is possible.

This shot was made with the 1.4 extender giving me an effective 1120mm lens. Still wont work with the 2x.

This image was sharpened 15% with Nik Pro Sharpener