I was also hoping to see a 100-400 this year as well. It would have been a good addition for me. Not sure yet if I wait, buy the current, or re-evaluate.

I had thought about the 70-200 f/2.8 IS II + 2x TC as you are thinking about, but I have the 70-200 f/4 IS and like the weight and cost. With the 100-400 it would have also let me add a 1.4x TC with the 5D3 if I wanted a little more reach. I'll probably have to map out a few options and see where the overall costs goes. I am not sure I want the size of the new Sigma 150-600.

current line up for my 5D3
* 16-35mm f/4L IS
* 24-105mm f/4L IS
* 70-200mm f/4L IS
* 35mm f/1.4L
* 100mm f/2.8L IS Macro

Overall, I like the spread I have. f/4 has worked for me and it lets me spread the money out across lenses. If I need faster, the 35 f/1.4 has been good. Next is figuring out what I want over 200mm.

Eric