The word according to God (Canon) is mentioned in this thread:
"Requirements for Auto Focus
Canon makes SLRs with TWO distinct Aperture requirements for Autofocus. (AF)
A: The 1 series can AF with apertures as small as f/8
Models include; EOS 1D, 1Ds, 1DMkII, 1DMkIIN, 1Ds MkII, and film cameras EOS 1 and EOS 3
B: All other EOS bodies require f/5.6 to AF
Models include D30, D60, 10D, 300D, 350D, 20D, 30D, and 5D
Film bodies = all of them except for the 1 and 3"
Note that this means the widest aperture WITH the TC, so for the 400mm 5.6 lens, you get f8 with a 1.4 TC.
So, the real question is how does the converter work on the type B body. Myself and others in this thread have been saying that it does work with the Kenko Pro DGX 1.4 model even with an 5.6 lens or slower.
As far as how much light is required with my 60D and 400mm 5.6 lens plus the Kenko TC, I guess the only trouble I've had is in the forest with dense trees, shooting birds in foliage. But that can be a challenge even without the converter sometimes. I wouldn't hesitate a second to get the TC and use it with a 7D plus the 400 5.6.