Spent some time with the ISO 12233 charts this morning (on my new Apple 27" Thunderbolt Display , which for some strange reason shows a lot more detail than the iPhone I was using last night) and the 2x III results in a pretty big IQ hit - the 400 II + 2xIII loses out to the 100-400 @ 400mm (yes, that's 800mm vs. 400mm, but that lets me compare it to what I have now), and the 500 II and 600 II with 2x III also lose out to the 100-400 @ 400mm. OTOH, with the 1.4x III, the 500 II and 600 II are still better than the 100-400, and natively they are much better.

So, I think that eliminates the 400/2.8 II from my list. I just need to figure out if the 600 II is actually handholdable for me, or if the small weight reduction with the 500 II makes that much of a difference. It's the difference between 12.7 lbs and 11 lbs - not sure if that's meaningful. Note that when I refer to 'handholding' the mostly means walking around with it on a Blackrapid strap and lifting for shots along the way. With my 100-400mm, I often bring a monopod (would need to replace the Manfrotto 234 tilt head with something like the RRS MH-02 for the increased load), and use that when I'm going to be in one place for a while.

Jon, I like your idea but I only have two kidneys...