I often use the 1.4x III with the 100-400 II. Mostly with a 1DX2, but sometimes with a 5D4. Works great! I'm shooting go-kart races tomorrow and will be using the 1.4x III for that.
Here's an example at 560mm:
Most of my birdpics are with 5DIV, 100-400II and 1.4EXIII, coming from sigma 150-600Sport I've got no regrets whatsoever.
Many of my Ecuador bird shots were with the 100-400II/1.4XIII, but on a 7D2. Many were seriously cropped too. I thought they looked pretty good.
Thank you for the info guys - Think I'll pick up a used one and give it a whirl before I part with the 150-600!