Tried the Kenko 1.4x(DG Pro 300 "red dot") very briefly on the 500mm when I had it - whilst I didn't try any tracking shots, the static images were super sharp. At 700mm I couldn't find fault in the test shots I took. I know the Canon mk3 TCs should be better but assuming I buy the 500mm, the Kenko would do for a while if I needed more reach than 500mm.
I guess it depends on how much you are cropping. Once I started using the 600 on my 7D, especially with the 1.4x TCs, birds got really huge in the frame. Sometimes I have to move back with some shorebirds, waders, and waterfowl. With the Kenko, you notice the IQ falloff as you approach the corners. Even midframe, there is some noticeable softening, and in the edges and corners, definite CA. I don't see nearly as much of that with the EF 1.4x TC III. In the deep corners you see some CA, but for the most part, it seems to have a flatter field, and IQ overall is much nicer. AF performance is good...not all that much faster (Canon really seems to slow it down on the 7D), but definitely more consistent...the drive just seems to work better.
If birds are still only filling the center of the frame, then the IQ difference between the Kenko and Canon 1.4x TCs isn't really noticeable outside of some extreme pixel peeping.