Thanks guys. It's difficult to tell for sure but I shot a lot of images with the 1.4x TC yesterday and my impression is that sending the 400 IS II back to Canon for the firmware update did help with the focus tracking. I was very pleased with the tracking of athletes that were accelerating/decelerating the way hurdlers do as they run and then jump then run again. Of course the 1D X did its job as well.
If you have the II and haven't done the update yet, do it.
Phil

























