First...I am pretty impressed by the 400 f/5.6. If you go back to the previous post it's 1,390/1,160/990 (Center/Weighted Avg/Corner Avg) is beating the 100-400 II's scores of 1,380/1,020/760.

Second...if I only had ~$4k more, that 400 DO II looks amazing. But, I'd probably shoot it at 560 mm with the 1.4xTC. I will saw, using the Sigma 150-600S last week, I was impressed how often I used the push-pull feature as the birds moved. There are benefits to a zoom when shooting wildlife.