Well, with all due respect, setting aside speculation about the effect of type and number of motors in the lenses, as well as about any effect of potential incompatibility of AF signals, here's my own experience using a wide range of adapted Canon lenses with A9 bodies via Sigma MC11 adapter over the past year. In short, with current firmware in both the A9 as well as in the MC11 (yes, Sigma has updated their MC11 fw as well), the adapted Canon lenses work pretty well for me. Not perfectly, but pretty well. I don't own the Canon 500/4, but I do have the Canon 200-400/4+1.4X which works pretty well with the A9 (even when I add a second, external, 1.4X to reach 784 mm at f/8). Yes, the A9 focuses pretty well even with that combo. I have also used a Canon 70-200/2.8 v2 with the A9, in very dim light, and again this works pretty well. And I have also tried the Canon 100-400 v2, and it also works pretty well for me, though not as well as the native Sony 100-400, which I own and use regularly. "Pretty well" is highly subjective, of course.
Of course, the A9 and A9II bodies should be very similar in this regard.
And just out of curiosity, I recently put a consumer-grade Canon zoom (70-300 v2, non-L) on an A9 via MC11 and got some decent shots with this combination, at the dog park (toss ball with one hand, shoot with other). The selling point: this is a very lightweight combo. I did miss a lot more shots than with my L lenses. But this wasn't a scientific test by any stretch...
Are any of these combos perfect? No, not at all. Certainly I get faster and more reliable AF when I attach a native Sony lens to the A9 (I have the 100-400 and 200-600, for example -- both great lenses, I find). Also, the A9's max frame rate drops from 20 fps down to 10 fps with any adapted lens, which certainly can matter in some situations. Most importantly, what I personally find is that initial focus acquisition by the A9 can be a bit slower with adapted Canon lenses compared to native lenses, but once focus is grabbed it's very accurate and also tracks movement well. (Dissenting views expressed here notwithstanding.) The Sony native lenses grab focus faster, which is worth the entry fee to me. It can make the difference between getting a shot and missing it.
I think someone here commented about how good Sony's 200-600 is, and I must agree. Yes, it's 1+ stops slower than the 600/4 but the A9's AF still works very well ("very well" being much better than "pretty well" btw). Even at 840 mm f/9 (ie, 200-600+1.4X), at dawn and dusk, the A9's AF still works "very well" (tested 2 weeks ago in Venice, FL). Although, depth of field is what it is... So, I'm still deciding if I want to get the 600/4 E-mount, which is $11K *more* than this excellent lens. Another way to look at it: the 600/4 is >6X the price of the 200-600 (someone here said 4X: wishful thinking?) That's a lot of dough for a bit more than 1 extra stop. Rumor has it that Sony will release a 500/4 in E-mount soon, but I expect it won't be much less expensive than the 600/4. Yes, in some situations the faster lens is needed for sure. But it's a highly personal cost/benefit analysis.
Coming back to the original topic, and with apologies to the Canon gods, nowadays I actually prefer shooting the A9 with adapted Canon lenses than putting the same Canon lenses on the Canon bodies that I own, for wildlife action at least. And I do own current Canon bodies (1DXII, 5DIV, 7DII) and have used them for years (ahem). Granted, this may be the result of my own incompetence. Namely, I was never able to master Canon's AF system (mea maxima culpa) -- I know Canon's AF works well for many people out there, and here as well... I tried (really, I did), but I was never happy with Canon's AF, personally. In my humblest of opinions, the AF of the A9 is just that much easier and more satisfying to use -- for me. Even with adapted Canon lenses... I know this is heresy, and I will likely be sent to Canon hell when my time comes (I shudder to think what that might be).
If I may digress a bit: I bought my first A9 about a year ago for about half the price I had paid for my 1DXII (the A9 was slightly discounted by then). I liked the A9 so much that I got a second. In addition to the AF, I really like the smaller size and reduced weight. I find the A9 can do everything the 1DXII can do, and more. But that's just me. I know many don't like the A9's ergonomics (I've grown used to that aspect), and the A9II is definitely improved in that regard. The customization of the buttons on the A9, especially with latest fw, is great: I rarely need to go thru Sony's menus, but when I do I actually find them quite similar to Canon's menus (that is to say, neither great nor horrible -- just necessary). Also, I am at an age where I need reading glasses so using the back screen on any camera is always a challenge -- being able to review images in the A9's EVF means I don't need my reading glasses anymore, when I go out to shoot! (I also don't really need the back screen, actually...) This really improves the whole experience, for me. Having all the shooting info, including live histogram, available in the EVF is fabulous. And yes, the A9's AF system never ceases to amaze me. Watching the little dot stick with my subject (usually attached to an eye!) no matter where my subject moves in the viewfinder is a real joy -- I can freely re-compose an image in real time while my subject is dancing about without worrying about keeping a focus spot on my subject, as I always worry with Canon bodies... And yes, this works (for me) on the A9 with adapted Canon lenses as well. At Bosque, in Africa... Another thing I love about the A9: complete silence. Bird/animal subjects have no idea I'm shooting, and neither do people at concerts and other social gatherings, nor fellow photogs nearby, of course... The silence is incomparable to so-called "silent" mode on my Canon DSLR's. Oh, and let's not forget: no viewfinder blackout at all at 20 fps... little symbols at the edge of the EVF frame indicate I'm shooting (these can be disabled), but otherwise I just follow the action with no interruption. By contrast, when I shoot with the 1DXII, the blackout from the mirror cycling at 14 fps actually bothers me now (looks like stop-action or slow frame rates from early motion picture days). End digression.
I stress that views expressed above are solely my own. YMMV. In fact, YMWV. I am fully aware that dissenting views abound, both here and elsewhere on the web.
NB: I have received no compensation from Sony
And all of my Canon gear is currently for sale (shocking, I know).
Armand