I was just playing around the other day, and put the 2X on the 100-400 II (with the 7D2). I sometimes use the 1.4X on it and have been pleased with the results, and wondered what sort of garbage I'd get with the 2X. Of course it won't AF, so I went to Live View to zoom in and try to focus, and I was surprised to see that it will AF in that mode. It's slow and probably not pinpoint accurate, but it never hurts to have these things in the mental database. I may try to find some use for it -- might garner an image for artistic processing. (But then, why not just crop a 1.4X image....)
Just thought I'd toss out the observation. Live View is a tool most of us have and probably rarely use. I usually only think of it for focusing the IR-converted 5D2.