Quote Originally Posted by HDNitehawk View Post
@Joel; I tried the 1.4 III versions a while back, and really didn’t see much difference in IQ. In sharpness the III version made the 500mm less sharp in the center and a just a shade sharper on the edges. Overall I didn’t think it was a worthy upgrade, in fact no upgrade at all, and I sent it back. I wasn’t completely convinced I had a good copy.

It looks like you have a good copy of the 2x III. You are right, Arthur Morris swears by the 70-200mm F/2.8 with 2x III extender combination; however he does put the sales pitch out for Canon. I might give it a try for the 70-200mm and if it works well with the 500mm it would just be a bonus.
You are right, Artie is a consumate salesman but there are many others who feel the Version III 2X is a big upgrade whereas the Version III 1.4X is not that much better than the Version II.

I have the original 70-200mm f/2.8L and the 300mm f/2.8L ...I have not yet tried the new teleconverter on them.