I was thinking more in terms of the 1.4x, taping the pins would be my only option with the 1Dx but still an option with a 1D IV which is not completely out of the equation. If I lost a significant amount of AF speed at 840mm were I will be mostly shooting at that would be a deal breaker for me.

You are right, from the ISO charts the 800mm seems to to be only SLIGHTLY sharper in the corner and actually are slightly worse in the mid frame and about the same in the center. I think the CA is the biggest differentiator, which DxO handles very well even when it's not profiled. To me the only reall IQ advantage is at 1200ish range were the 800mm handles the corners better, but I don't really have anything that far in the corners very often. And even then it's not a huge difference.

John.