Autofocusing at f/8 with Select Third-Party Extenders

Many people were unhappy that the Canon EOS 1D X could not auotofocus at maximum apertures of f/8 like previous 1-series bodies. We received a few reports that the Canon EOS 1D X can autofocus at f/8 using the Kenko Teleplus PRO 300 DGX 1.4x AF Teleconverter (Aurthur Morris posted on the subject recently) and we decided to run a few small tests to confirm.
 
We tested a "blue dot" (current) version of the 1.4x Kenko Extender with the Canon EF 400mm f/5.6 L USM Lens using center focus and various camera bodies focused on static subjects. Here are our subjective results:
 
Autofocus Performance

• Canon EOS 1D X: AF worked and was fast with almost no hunting. However, metering was consistently off by +1 stop.
• Canon EOS 5D Mark III: AF worked and was fast with almost no hunting. Metering worked as expected.
• Canon EOS 5D Mark II: AF worked and was reasonably fast with a little hunting. Metering worked as expected.
• Canon EOS 7D: AF worked, but the camera had a hard time locking focus. Metering worked as expected.
• Canon EOS Rebel T4i / 650D: AF worked with a little hunting. Metering worked as expected.

Please note that we did not test with moving subjects and did not test image quality; results may vary depending on your situation. Other third-party extenders may work similarly.
 
Update: The EOS 1D X was originally tested with version 1.0.2 firmware. I updated the firmware version 1.0.6 and retested. Performance remains as reported above. Also interesting is that all 1D X focus points remain active (though periphery AF point use results in more hunting).