I have the 2x mkiii. AF takes a hit with some of my lenses and I lose a lot of focus points. My best lens with the TC is my 300Lf4, AF remains quite good and I get 9 focus points on a 5Div. I have the 100-400 Mk1 and I don't get any AF with that on my 5D. AF will work in liveview, with the mirror out of the way.
I bought a little Canon M5 recently, as AF will work on that with all my lenses and the TC as there's no mirror. There's no substitute for aperture though, with some of my lenses, I'm up at f13, which is close to useless in most wildlife situations. IMO, you will always be trading something for extra reach with a TC. They have their place but not for everything.
1.4 seems to be the best compromise.