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.
Also consider the 500mm f/4L IS. It's heck of a lot bigger than the 300/2.8 or the 400/4 DO, but it's lighter than the 400/2.8 and 100mm longer, too. Most who use the 500/4 consider it a handholdable lens, which the 400/2.8 is not.