Autofocus

[Chevi]

I intend to use the Canon 100-400 Mk11 with a 1.4 tele converter, will the AF work ok? (using a Canon 7D Mk11)

[cicopo]

It does work but most likely center AF point only and it will be a bit slower.

[ebiggs1]

"Works" is a relative word here.  It probably will work but it will not work well.

Have you considered, you already have a tele advantage fo 640mm with that lens.  Maybe the only place where a EF, or full frame, lens has an advantage is on a crop body.  640mm is going to be a challenge to deal with.  Nearly 900mm is really tough. Strong tripod.  Everything has to be right for the shots to be worthwhile.