I can't justify f5.6 with the TC is engaged for the same price as the 400mm f2.8 II. There must be a lot of R&D Canon has to recoup on this lens for it to be priced at $11K.
I agree that the guess of $11k is a bit steep, but its tough to compare it with a 400 f2.8. I use a 600 f4 and a 300 f2.8 frequently, and depending on where I plan to shoot and what I am shooting will help me to decide which lens to bring.
Its just too much gear for me to bring both. This is why the 200-400 would interest me. It would give me more flexibility when I am out in the field and allow me to cover a broad range of zooms. 200-560mm. Also, unless I am shooting subjects that are very close to my tele lenses such as birds, I often stop my lens down to f6.3, f7.1 or f8 for a little higher IQ and longer DOF with larger wildlife. Of course DOF considerations come first but I do find that I am often not shooting wide open.
Also, my 600f4 (NON IS version) weighs 13 lbs. The 400 f2.8 weighs over 11. Both are an absolute bear of a lens and need a tripod to shoot with. The 200-400 will likely come in much lighter. Maybe in the 7-9 lb range. It could even be lighter! (Afterall the new 600 f4 is only 8.5lbs!!) While this would still be a bear to shoot handheld it would certainly be do-able and with a monopod would be a breeze...
All said and done, if the IQ compares to either of my long lenses at their respective focal lengths (560 is pretty close to 600) I would be inclined to consolidate my kit and sell the 300 and 600 to put towards the 200-400.