AF using 100-400 + TC

Are there any Teleconverters I could use with my 20D w/100-400L/IS and retain its AF function?

It would probably work with the (non SP) Tamron 1.4x. Note, I say "probably". The 20D is only rated to focus with lenses which have a maximum aperture of f/5.6 or larger so the resulting f/6.3-f/8 lens is outside specification. Performance in low light may be marginal.

I have the 1.4x Kenko 300 Pro TC and it doesn't keep AF : f/6.3 is definitely outside specification for a 20d body.

https://photography-on-the.net/forum/showthre​ad.php?t=41922

I have el cheapo Kenko 1.5x non-pro non-anything TC (wait, it says SHQ, might it mean Super High Quality?), autofocus works in almost all situations (essentially, when the lens is able to focus without TC, it does so with TC too). That's with 10D, which is supposed to have worse autofocus than 20D.

With aperture closed down one stop, I find the combo very usable. Closed more, it becomes even better, but one usually does not have such luxury, bearing in mind the high shutter speed necessary to avoid blur.