I think the 1.4 is a must have for full frame owners. Agree 400 is out of the question for just about everyone. Unless your sports illistrated that buys them for their staff to use at the olympics
I use the 1.4 all the time with my 1Dx2 and the 300 and there is negligible loss in performance as compared to the 7D2. Even with a 1.4 the 1Dx2 Blows the 7D2 out of the water with AF performance and tracking. So much so that Im only using the 7D2 for reach needs and stationary and larger subjects like elk and bears, coyotes and big birds that are not on the move. Its night and day difference with 1Dx2 and 7D2 tracking capabilities.
I have the Sigma 2.0 TC and it seems to perform better than the canon in my opinion. I rarely use the 2.0 and only break it out when I have other choice. The grizzly I got at Yellowstone with a 7D2 and a 150-600 comes to mind. It was the only way to get me close. I was a6 600,, on the lens, with a 2.o TC, and the 7D2 manual focus....Got him but nothing i would print. This is the shot with the 7D2
This is also a fairly large crop. But you could print bigger than 8x12 for sure