Kenko changed how the TCs report when they moved from the DG to the DGX series. The DG series (and I owned a 2x DG model) would disable autofocus if it went above f5.6. However, the new DGX series TCs do not disable autofocus.
Ok, this contradicts what I've read before, but I guess I believe you since you have a 7d. To make dead sure: We are talking about the "Kenko 1.4X PRO 300 Teleconverter DGX for Canon EOS Digital SLRs", are we: