After reading Authur Morris blog, I went for the Kenko 1.4 Teleplus for use with my 70-300L f4-5.6 and Canon 5D3. I'm happy to report that the 5D3 retains all 61 points AF. The AF at F8 isn't bad at all and the IQ is useable. I have also a 300F2.8 + 1.4X and 2X, but for a walk around lens, the 70-300L is much lighter and the Kenko 1.4X does a pretty good job. Will test some more in the weeks to come. The attached photos are shot at 1/2500, f8, ISO 1250, 420mm. The focus is spot on as in the second photo.