A little more follow-up:
I shot over 2,000 shots this weekend with the Sony a9, Metabones EF-to-E T-adapter MkV and my Canon EF 100-400mm f/4.5-5.6L IS II, with and without my Canon EF 1.4x TC-III. Subjects included Painted Lady butterflies, egrets, heron and deer. I took loads of test shots of passing cars, bicyclists and runners.
As my subjects changed, I changed the AF pattern from "Wide" (using all 693-AF points), to "Center" (using 9-AF points in the middle) and "Single-Point Adaptable", which allows you to steer a single point with the joystick.
Shooting butterflies on a windy day, with both the butterflies and wildflowers moving in the wind, Wide couldn't decide what to focus on, so I quickly switched to Single-point, for dozens of in-focus shots as below:
Painted Lady Butterfly by
David Stephens, on Flickr
I was shooting white-tail deer crossing the road, using Wide pattern. This doe allowed me to get close for a portrait and I left the pattern on Wide:
Doe Eyes by
David Stephens, on Flickr
When using Wide and the with a subject that's not real contrasty, you have to move the AF grouping to the subject. I found myself able to "will" the points onto my subject. I didn't use the joy stick and I can't really explain how I do it. I see where the AF points are "dancing" in the EVF and move the aim enough to get them where I want them. I did this with the doe. Face detect and eye detect are on, but it does not detect animal faces. I know that it does detect human faces, because I shot runners and bikers and a square would lock onto their faces.
Shooting a contrasty subject with Wide AF pattern is easy as pie. The system locks onto the nearest contrasty object and tends to stay locks, whether the subject is still or moving. The dancing AF pattern tends to go to the head, but can move from head to body, so I would "will" it to stay on the head, with subtle movements of the camera.
Catch by
David Stephens, on Flickr
Tracking cars, bikers and runners with Wide AF pattern was also easy as pie. After initial lock-on, I would get an infinite number of in-focus shots as the subjects moved toward me from 8-mph to 35-mph. This was not at all challenging the system. I didn't keep any of those images to share, so you'll have to trust me.
Occasional problems including missing focus by a long shot and requiring a second or so to regain focus, if ever. It's critical to prefocus. As clouds came and contrast dropped, there were times when prefocusing was hard, particularly with the 1.4x TC attached. I recycled the body once or twice, or cranked the manual focus ring to deal with this. This was only an occasional problem. If a bird is small in the EVF and you don't grab focus right off the bat, the hunting was generally not successful.
Once in a while, I'd see AF dancing right on top of the subject, but later inspection would show them OOF. If I was taking a series, the whole series would be OOF. With Canon, you can start OOF, but keep battling and then end up with a shot or two in focus. I had no such luck with the Sony. It was either all or nothing.
Bottom line is, I prefer my 100-400mm on the Sony body vs. the Canon 5D MkIV. There's learned art to using both.
Full disclosure requires me to state that I'm going to order the Sony 100-400mm G Master lens and the Sony 1.4x TC. I'm Jonesing for the 20-fps and even quicker reflexes and reliability; HOWEVER, if I couldn't afford to do that, I'd be very happy with the Sony/Metabones/Canon combo.