A 600 mm.lens, I would like it but I can not.
For many wildlife photographers, this is a real problem, in my case I could but I will not.
Why?,Cause I do not think it is right for a hobby to spend on a lens as many families spend to feed a whole family for a year.
So what?
Now for the video mode there is a great news.
With the magic lantern firmware for Canon you can record a crop of 3X quality RAW 14 bit, almost as if instead of a lens of 400 mm had a lens of 1200 mm.
This opens up new scenarios in the world of nature documentary and I believe that soon we will see the effects.
I attach some links for those wishing to experience.

Magic lantern site:
http://www.magiclantern.fm/

Sample from youtube user:
https://www.youtube.com/watch?v=dQ51rE_ZUgo