OK, a trip into the archives and I found this on page 50 of the EOS 3 user manual:
"(3) With the following lenses, if the maximum aperture is
anywhere from f/6.7 to f/8 when an Extender is
attached, the center focusing point will be sensitive to
horizontal lines only. The other focusing points cannot
be used for AF.
• With Extender EF 1.4x:
400/5.6L, 500/4.5L, 100-400/4.5-5.6L IS
• With Extender EF 2x:
300/4L, 300/4L IS, 500/4L IS, 600/4L,
600/4L IS, 70-200/4L"
So it is not unreasonable to expect a 5D mkIII to af at f8 with the central point (or points). Not unreasonable to expect that of a 7D mk II either, I say.