Perhaps Sigma will be out with their new 120-300mm f/2.8 OS Sport lens before long and throw in a 2x converter with it.
I personally got tired of waiting and got the 300mm f/2.8L IS II: cheaper and works a charm with both v3 teleconverters.
I have a 300 f/2.8 IS and both V2 converters and the image quality is outstanding. But that's with a 300 prime, one of the sharpest lens that Canon makes. The issue I have is that I often shoot in dusty environments and dislike taking the lens off to change/add/remove TC's.
I'd rather have a 200-500 f/4 and a 1.4X TC than a 120-300 f/2.8 and a 2X TC. I think the image quality and focusing speed would both be much better.