Id keep the 2x away from the 100-400 tbh, i was really disappointing with my 100-400 when I put the 1.4x extender on it. I ended up selling the 100-400 and replacing it with a 300f2.8 simply because the IQ wasnt anywhere near what I expected. (Mine might have been a bad example, it certainly wasnt great on my 1series, but on the 20d it seemed perfect)
From all ive read I think I will end up getting a 2x for only when walking a few metres closer to the subject isnt possible. Ive found that primarily using a prime lens I already do this, whereas with the 100-400 id be lazy and just zoom around the scene from a fixed point.