The 70-200 mkII carries the 1.4x very well, and the 2.0x well. I don't use them so much now as have a 300 f/2.8
Depending on what you're using 400mm for and how often will dictate if it's suitable. The AF is usable but takes a big speed hit. Stationary or slow moving subjects are ok, but anything faster you need a little more patience and thought to get the most out of it. Also image quality is usable at f/5.6 though tends to be ok at f/6.3 and better at f/8.
Here's one with the 2.0x at f/5.6. Can't find any others at the moment as I'm at work & just picking what's out there in cyberspace. The mkIII versions of the extenders are supposed to be a touch better than the mkII versions. These are all with the MkII's
The 1.4x on the other hand is very usable, with minimal image quality or AF speed loss.
http://www.ausphotography.net.au/for...reat-Bowerbird
http://www.ausphotography.net.au/for...ater-Crocodile
Personally I'd use the 2.0x for occasional use and err to the 1.4x, and get closer/and or crop a bit. Another option is to look at the 100 -4000 or the 400 f/5.6 as a dedicated longer lens if you intend to be at 400 regularly. But in saying that, I have seen a number of posts around the net using the 70-200 & 2.0x MKIII with good results.