Upload & Sell: Off
I went out and tried the 100-400L with a taped 1.4x tc. I tried it indoors, as well as in decent light outdoors.
For non stationary objects, the focusing is good and accurate. a little slow, but over all not bad. overall much better than both my 5d2 and 7D with TC. Spot focus also works.
Here they are, at reported f stop of 7.1 (f10), iso 1600,
the tracking, is not great. you need a reasonably large subject filling most of the screen. its really nice to use a lot of autofocus points (all of them if possible), or expansion or it will have some difficulty tracking. and forget about any really fast moving birds, but honestly the bare 100-400 is not the fastest lens in the world. but large target, reasonably slow moving, very good.
here is 20 shot sequence of an approaching pelican. every shot is in reasonable focus. click to magnify
here is one of the shots, large