I was told both lenses at 400mm f5.6 pitted against each other, the 100-400L would smoke the 70-200mk.ii + 2x iii combo. So far this one test shot shows both very close, I am preferring the 70-200+2x due to better shadow detail: 100-400 has more contrast though... Open both files and pixel peep please.
Looking at the thumbnails the 100-400 looks better, but at pixel peeping levels the 70-200 2x looks better.
Both are 100% crops taken on a rainy evening under high ISO.
NO pp was done to the RAW before converting to jpeg in CS5.