The opportunity to photograph these birds in Tokyo came as an unexpected bonus. When I first encountered them I did not have my 100-400 with me but I had my 70-200 f2.8 and the 2x teleconverter. The next day I went back with my 100-400 and the gimbal and tripod.
First eleven images: Shot handheld using Sony A9 with 70-200 f2.8 and 2x teleconverter at f5.6. The first image was 1/2000 of a second and the rest were 1/4000 of a second (these birds are crazy fast). Traditionally, teleconverters have really compromised image quality and especially so with 2x teleconverters. The shots here are proof that Sony has done a fantastic job with their 2x E-mount teleconverter. To be fair, I had a lot of light in this session and the autofocus, even with the A9, was a little slower but the images are still very sharp.
Next twelve images: Shot on gimbal and tripod using Sony A9 with 100-400 f4.5-5.6. All images were shot at f5.6. The first image was shot at 1/3200 of a second and the rest were at 1/2500 of a second. It's not a controlled test since there was also a lot less light, but softer light, when I took these images, but the sharpness of the images from the previous session compares well to these ones.
Last nineteen images: Shot on gimbal and tripod using Sony A9 with 100-400 and the 1.4x teleconverter at f8. Shutter speeds varied from 1/1600 to 1/2500 of a second. The image quality of Sony's 1.4x E-mount teleconverter is superb. It does affect autofocus speed a little bit but I notice this a lot more when using it with the 70-200 f2.8 than with the 100-400 and only when there is really fast non-stop action.
Unfortunately I didn't shoot any of these images using the 100-400 and the 2x teleconverter.