Doesn't matter if your object is going 200 or 300+mph. it's not hard for any decent action camera to track fast-pace subject because the AF is engineered to predict and calculate object going in straight path, constant speed with algorithm that hard to miss once it's locked.
Straight path? You've never seen one of these fly. The paths are nowhere near straight, and they can out-turn and out-accelerate birds easily.
For photographer, you just follow the motion - not much of a challenge like shooting the big black cormorant against the sky. You and the camera would pick it up and lock the focus in with ease.
I've never shot anything more challenging than these. They are fast, erratic, hard to follow, and totally unpredictable. Birds are a cake walk by comparison.
An erratic fllyer have no direction, take off is sudden and you can't predict the direction, vertically or horizontally motion and It happens in sub second. You don't even have time to think, or get the viewer finder lock in the subject. A few good example would be diving duck surface and take off, small hawk/falcon hunt for preys, shore birds in flight.
Sure, i can show you a few lucky shots to prove my point - Hey, i got it with 2x ! it's tack sharp but what's the point? 1 out of hundreds with no consistency just to win an argument is really what i do day in and day out?
I get about 95% in-focus with the 2x.
No, i don't use 2x for fast action unless i got no choice because it's unreliable.
I have a 100-400L, and it's no more reliable. Less, actually.
and yea, that bird is easy because cormie behaves like plane.... but do try it with your 2x against grey tone busy background.
Here's a green-and-brown camouflaged plane against a green and brown background.
Not busy enough? How about this one:
--
Lee Jay