Here are some images of Mount Spokane from my back deck for a comparison of my 100-400mmL at 400mm with 1D MK III and Canon TC mk II's. I've cropped them all to the same view. With no TC, thats 100% so the image is smaller. They have no post processing other than my lightroom defaults, so obviously they can be improved.
I feel that the 1.4X TC provided the best image, I tried to avoid any motion and used a heavy tripod with underweight, however, with the long TC's, there may have been some vibration, particularly with the 2X. The top of the mountain is several miles away, so there is a lot of distortion coming from 7 miles of air.
Nice comparison. Could you also post the f-stops used or do you have a stopped down image of the 2x? In my experience the 2x "wide open" doesn't work very well.
Also i have the 2nd hand impression that there are old and new 100-400mm lenses which perform differently. So if you have an old one it wouldn't work as well.
The lens is fairly new, I love it. These are difficult shots due to the distance.
Reported Settings. These are what is recorded in EXIF and, in the case of stacked TC's the f stop is wrong. I used AV setting of f/11, but the aperture of the lens would have varied according to the TC with it being stopped down more without a TC
Also, it was a winter day and the light varied from shot to shot so the shutter speeds varied according to available light.
No TC - 1/2500 sec f/11, ISO 800
1.4X - 1/3200 sec f/11 ISO 800
2X - 1/3200 sec f/11 ISO 800
1.4 + 2X - 1/2000 sec f/11 ISO 800 (not the actual equivalent aperture)
It might be interesting to redo them with my 1D MK IV, but in our near 100 degree weather, the air is too turbulent, and the 1D MK IV is going to be sold soon.