I'll look into the 300 2.8 and the 400 DO. My main concern with the size of the 400 f/2.8 is traveling with it. How can I get that, along with several other lenses and a body into a carry on bag? I wonder how the other guys get it on a plane or maybe they don't bring many other lenses.
I travel with both a 600 f4 and a 300 2.8 all the time. I put them in a backpack along with 2 camera bodies, several smaller lenses, teleconverters, and binoculars. I use the Tamrac 787 backpack :
With the back straps pushed away it fits in even the small overhead compartments of commuter flights. My small laptop fits in the front pocket. I usually remove the lens hoods for the big lenses and put them in my suitcase to save room. I wouldn't worry about a 400 2.8, especially if you're not taking a 300 2.8 also.