I have done as much research as possible and have gone ahead and ordered the Sigma 500mm f4 lens. One of the things I noticed in reading many reviews of the lens is that very few actually gave the lens a good work out...and in particular only a few used and fine tuned the lens with the Sigma dock. That fine tuning in many cases improved the focus speeds to be equal with Canon V ii. And the tuning of the IS system and fine adjustments to focus (AFTER the lens was AFMA'd to the canon Body) made positive differences. Read the link posted earlier form Brad as to his experiences with these adjustments.
I am very pro buying Canon big whites, but as I ordered my Sigma from Australia, I got it for USD 4500. between USD strength, rebate of 10% Aussie tax and a discount from the seller, that made a savings of nearly $4500 USD over a canon ver. II 500!
I will not get the lens to my home in Panama for a few weeks, but will post some photos from a 5D4 ASAP...