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/5.6L USM Lens using center focus and various camera bodies focused on static subjects. Here are our subjective results:
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).