As regulars to the Bird Portraits and BIF threads know, I am very enthusiastic about both the 400mm DO II and 100-400mm II. Yesterday, I posted some sharp images from the 5DSR + 100-400mm II + 1.4xTC in both threads. The 400mm DO II + 1.4xTCIII is even sharper, and is as sharp as the 100-400mm II at 400mm as found by my experience and the MTFs posted for both on Lenstip. The 400mm DO is tack sharp from edge to edge on FF. My latest copy of the 100-400mm II is very good as well, but an earlier copy was definitely softer. The 400mm DO II really comes into its own with a 2xTC when it is still very sharp at 800mm. If you are going up only to 560mm, then my preference is definitely for the 100-400mm II because the minor sacrifice in IQ is more than compensated for by its flexibility and light weight. But, if you need to squeeze out a useable image from very small birds at medium distances or larger ones far off, then the 400mm DO II + 2xTC is the choice.