Ok this are some of the pics taken with 100-400L + Kenko 2x DGX with my 5D. All shots are hand-held and in AF mode.
Not a good day to do testing, overcast sky and drizzling in between. ISO used range from 400-1600. Because my camera is fitted with the Ee-S focusing screen, it was almost pitch dark before focusing was locked. Ok pics first, all cropped quite abit except #1
#1
This one I thought it looks ok, but not at tele-end.
TV: 1/500
Aperture: 9.0
ISO: 1600
Focal length: 200mm (At short end)
#2
This was taken within the vegetation, while walking on the mud track, so it's pretty dark.
TV: 1/250
Aperture: 11.0
ISO: 1600
Focal length: 800mm
#3
Drizzle stopped abit. This was taken without the TC.
TV: 1/200
Aperture: 5.6
ISO: 400
Focal length: 400mm
#4
It gets tough again with the rain.
TV: 1/150
Aperture: 11.0
ISO: 800
Focal length: 800mm
#5
The only BIF I gotten, rest all buang due to user mistake. FYI, it's the first time I try BIF. IS off.
TV: 1/500
Aperture: 11.0
ISO: 1600
Focal length: 800mm
Conclusion: The Kenko 2x can AF on the 5D. However the AF speed is real real slow. Saw an eagle in flight but my pics came out blurred as I did not managed to lock AF in time, and also because of user error. Those with Ee-S focusing screen on camera will find it tougher to use it. However given it's price, which is relatively low compared to the Canon's, it is good value for the times you need it, though I m pretty sure mine will not be used much since I am not into birding.