The limit with a Canon TC is f/8. I'm not sure how a Kenko 2x would do with an f/5.6 lens, but given the optical hit with the Kenko 2x (much greater IQ detriment than the Canon 2x), I'm not sure it's a good option anyway. Note that's for phase detect AF - contract detect AF works at f/11.
It would also be a game changer by being approximately the same size as the EF 1200mm f/5.6. I doubt we'll see such a lens...