Hi Richard,
The 600 + 1.4X (840) on the 5DMKIII was just about perfect for hummingbirds. I was shooting them from around 20', so a decent sized image in the frame but still enough field of view to track them with a little success and I'll get better at this part. I will be trying the same location with my 300 2.8 II w the 2X tele III this weekend (the 600 is on order now). If I can get a little closer, I'll have a comparable field of view. Shooting this lens combo w the 5DMKII was not very successful - the AF just wasn't up to it. My experience with the IQ of the 300 + 2X has been great and the close focusing is very cool if you can work from a blind etc... Having both the 300 and 600 is a luxury, but I can backpack w the 300 and converters while the 600 will be great for more singular photo activities. I'll continue to debate adding a 7D to really push the apparent focal length (could just about have bought one for the price of the two rentals...).
Thanks for the suggestion about Bob's HB thread - I'll give it a look. My shots this w/e were all natural in our perenial garden, but we have feeders around the house where I could place a photogenic bait flower and stage a multi-flash shot to better freeze wing motion. Always a new challenge.
Cheers