Well I had a chance to extensively test the 120-300 with the extender in very good light. It definitely, even in good light, loses a little bit of responsiveness in AF, but not that much if you have good light. Unfortunately, almost all the good shots I got with the full reach of the extender were of other kids (not mine), so I await permission to post from parents to show full reach. For some reason, my own son and I were having an off day with each other.
At least he finally got better fitting pants befitting an all-star. This shot didn't use the full reach with the extender, but it was on the lens:







