The 400 2.8 is aimed at sports sideline shooters. They are normally too big and heavy to use for BIF. Sony really needs a 500 f4 for their system, but I haven't read anything about one in development. I also use the Sony 100-400 with very good results, and with the 1.4 TC you get a 560mm f8 lens, which works very well.
Osprey #90 by
lennycarl08, on Flickr