I wanted to get a supertele later this year but the purchase has been set back by a few months. I was not expecting much from the combo but it performed reasonably OK and I feel that I will be able to manage to shoot the duck migration this winter.
I'm wondering whether it would be a smarter idea to get a 2x TC as well. I could use it on the 70-200 II and get almost similar results as the 100-400 while getting the 560mm reach on the 100-400 + 1.4TC mounted on the 5D3.
BTW, I didn't find AF to be too much of a problem - but then, maybe I was not expecting much anyway