I have the 70-200 f2.8 IS and the Canon 2x II extender. I have to say that I was quite impressed with both the IQ and the speed of the AF. To be honest when I bought the extender I really was expecting the worst, but ended up being very happy.
Just got the Kenko 2x TC DGX. On 5D MkIII, it ONLY works with 70-200 II, not anything else, which I expected since my Kenko 1.4x only works with 70-200 II as well. Kenko can keep denying compatibility problems with 5D III and put their head in the sand, but problem exists.
On 70-200 + 2x TC, probably cropped out 80% of the photo. It was at dusk, so iso2000.
DZ3C3817 by drjlo1, on Flickr
Nice level of detail considering the 2x converter. There is quite a lot of visible noise, though, did you push the exposure in post?