Mt Spokane, I appreciate your input. Again, I need the Kenko because it's the only TC that works with the 70-300L; Canon TC's are not an option. So, I have to decide if I need a teleconverter for my 70-300L more than I need properly AFMA'ed lenses. Tough call, especially because my 70-300L is so sharp, right out of the box.
spturtle, here was my sequence: I attached my EF 400 f/5.6 lens and ran FoCal to get an AFMA of +7. I then attached the Kenko TC and previously AFMA'ed 400, thinking I'd AFMA this combo next. That's when things went nuts. Once I realized my only escape was popping the battery, my only option was to DISABLE AFMA so I could use the TC + lens combo.