[VinnyP]

I guess it's all about image quality wide open. I haven't shot the Canon but certainly with the Nikon the 600 F4 is very good wide open add a Tcon even a 1.4 and it really really benefits from stopping down a stop if possible (and that's not the first thing you'll want to be doing at that focal length). Having said that 800 5.6s have been around for a long time and are not supposed to be as difficult to manufacture as a 600 F4 but thats before the days of IS.