I just tried focussing on closer subjects. The AF on 5DIII and 70D both failed, but the liveview on the 70D was spot on. So, perhaps this is the reason why I thought previously the tube didn't work? It makes me really glad I bought the 70D and sold the 7D because the latter had very poor AF on the 300/2.8 + 2xTC whereas the 70D is very good, and it has amazing liveview focussing.
I never got on with the 300/2.8 + 2xtc on the 7d. It was not workable for me, just too slow for wild life, but it should always focus using normal AF. I used to turn off the search for focus option. Or it would take ages to focus. But its great on the 5d3 or 1dx. In fact its probably my fav lens on the 1Dx and very often hand held at that.
But I don't think I would ever use the stacked converters for wild life. The IS is best turned off and the whole set up has to be well supported. Due to the lack of light you are going to need such slow shutter speeds its not really what you want for fast moving wild life in poor light. And that's always what I seem to be taking photo's of.