I've been reading Arthur Morris' blog for around 9 months now, and particularly heavily since around May. He is an extremely talented bird photographer, well-known for his use of teleconverters and f/8 AF in Canon 1D series bodies. Since the advent of the 1D X and the first time ever that the 1D series from Canon does not officially support f/8 AF, Art (along with many other professional bird and BIF photographers) have naturally looked for a way around the problem. You can read about arts experiences with the Canon 1D X, Canon EF 800mm f/5.6 L IS, and Kenko 1.4x Teleplus Pro 300 DGX teleconverter for functional 1120mm f/8 that produces sharp, professional results:
http://www.birdsasart-blog.com/2012/08/30/mega-amazing-discovery-800-f5-6-1-4x-tc-canon-eos-1dx/
http://www.birdsasart-blog.com/2012/09/02/100-proof-and-lots-more/
Seems the 1D X works superbly at f/8 with the Kenko TC.
http://www.birdsasart-blog.com/2012/08/30/mega-amazing-discovery-800-f5-6-1-4x-tc-canon-eos-1dx/
http://www.birdsasart-blog.com/2012/09/02/100-proof-and-lots-more/
Seems the 1D X works superbly at f/8 with the Kenko TC.