Just some Real World experience to add to the mix: the 7D will AF usefully quickly for bird photography at f/8 (eg Canon 100-400mm and taped or non-reporting 1.4x TC) off the central AF points. Use it on the peripheral points, and it's actually not bad at all.
For example - 600mm, handheld, (Kenko non-reporting 1.5 TC), peripheral AF point on the bird's eye.
The 70D (mine, and that of at least two other users that I asked to test this) will AF at f/8 on all AF points. Again, it's not lightning fast, but it works.
I put Keith's suggestion to work and the AF is pretty fast with my 7D with the 100-400 and a 1.4X non-reporting TC. You need decent light with nice contrast in your subjects. I got some nice shots with the sun behind some clouds this afternoon. Heavy shade was a bust as was shade with the sun behind clouds with a low contrast subject. Moving subjects are an issue when coming more or less directly at you or heading away. I did not get an opportunity to try with subjects moving in a more parallel vector.