I have been out shooting a red-tailed hawk behind my house for several days using my 200-400mm and 1.4X TC. I am finding that using the AF-On button for focus takes a lot of coordination. I have used AF-On now for several years and it works fine for static shots...like landscapes where I can focus and recompose. However, with fast moving birds, there are just too many things changing at once and my fingers get lost pushing buttons. I may try using the shutter button for focus today, go to 9 points AF and go with long delay for lock on.
I am trying to reduce variables and increase keepers. I have given up on manual focus. That just added more variables.
I am not sure if VR helps or hinders. It definitely improves sharpness for static shots. I have proven that using my FoCal testing. Just not sure it does the same for moving targets when I am panning.