I ran a completely unscientific test today comparing the Canon 1.4x TC III to the Kenko C-AF 1.4x Teleplus Pro 300 DGX. Both shot using a Canon 400mm f5.6 for 560mm on a 5D MkIII, center focus, f8, ISO 400, 1/640 sec. Auto-focused, of course. I took numerous shots with each and chose the best of the groups. I don't think there's much question which is better.
Canon
Kenko
The Kenko does allow f8 AF on the 6D and works with any lens. The Canon will not allow f8 AF on the 6D and will only work with certain lenses. So there's the trade-off. Looks like I'll need to pick up a Canon 1.4x TC III for birding, though.