Playing around with the 400 5.6 + 1.4x TC III to see how the f8 AF works. Not too shabby. But f8 and 1/1250s on a gray day is tough.
On a side note, I had forgotten that I had taped the pins in the 1.4x TC so my 6D would AF. My first few shots registered at f5.6 before I realized why I wasn't seeing f8. It didn't make much difference speed wise, but I had more focus points available to use when it was showing f5.6. I'm not sure how well they would have worked, though.