Well, I used the 1.4TC yesterday, so some comments there:
1. The size is excellent, extending the camera by just 16mm (compared to 26mm on the Canon 1.4 IIII).
2. It is a very important piece of the puzzle for Sony for now, because native lens options essentially end with it at 560mm/f8. The 2x is not a real option on the camera, because autofocus past f/8 is not PDAF.
3. Good news for Sony: The TC on the 100-400 GM has a smaller hit to autofocus speed than the Canon 100-400LII + 1.4 III.
4. Bad news for Sony: like the 100 - 400LII + 1.4 III, it's a pretty terrible Bird in Flight combination in terms of autofocus speed. But then again, the Canon combination isn't very good for birds in flight either -- not so much tracking the bird, but getting the initial AF lock, if you only have a couple of seconds and you aren't already focused on or near the vicinity of the bird.
5. The only way I could get it to autofocus was to AF on some trees at about the same distance at a lower zoom, then point it at the bird, and then letting it do its thing. Then, following the bird (using a small zone AF, like center + expanding).
6. I photographed 105 shots of two eagles circling in the air. I was very lucky: they circled for a long time. If you've ever shot eagles, when they do this, it's very easy to focus on them, because their flight pattern is relatively slow and predictable. Exposure setting was something like 1/2000, f/8, ISO 320. I was using a monopod with a sirui tilt head.
Literally ONE was in "best" focus, which wasn't really perfect, but was probably as good as I'd get with AF. TWO were very close after artificial sharpening. FOUR more were close enough to be considered good focus after artificial sharpening. 98/105 shots were garbage.
For reference, on a 6DII, my shot percentage would have been something like 60%+ in perfect focus, with or without an extender.
7. For bird portraits in a situation where the extra 40% reach lets you take a well-composed shot where the subject fills up the frame, it's excellent. In my opinion, better than the Canon, because you can focus magnify, manually adjust, and guarantee a perfect focus on the eye much easier than you can on the Canon.
8. Whites against contrast solids had CA that don't correct in post (without manually photoshoping it). Slightly OOF whites had bad CA, worse than without the extender. It is also small enough that if your subject fills up the frame and you can resample that image down to typical distribution sizes from the original 42mp, it will disappear.
9. IQ was generally pretty good, but it definitely suffers a little. If I could split the difference and get 20% closer to the subject, I would rather do that than to use take the extender, with the 40% magnification. This, based on some heron shots (though the extender shots were in a different location as the non-extender shots). Note that this is no different than the Canon.
Attached is the best shot I could get out of 105. It was photographed using uncompressed RAW (so, 85MB file), 1/2000, f/8, ISO 320. It is cropped, but not resized, and sharpened as best I could in LR.
This image has a long edge of about 1800 pixels, so it's not exactly a deep crop, though it's still a significant one. That said, I am able to get very good 1500-2000 pixel bird crops out of a 6DII with the 100-400LII with or without the extender, definitely superior to this one. But I will try some more, and report back.