5.6 is too slow for a prime....... like the 800-5.6?
the 800 f/5.6 too slow ? wow i guess you've never tested it
this new 400 would be great if it's a weather sealed lens with the IS II (and a price under 2k of course)
but if the new unofficilay announced 100-400 f4-5.6 L IS is as good as the 70-300 f4-5.6 L IS in term of sharpness and quality of the image the choice will be a tough one !