Can't wait for Part III of this series.
I just bought the Nikon 200-500 in spite of its weight because its VR is far better than the Sigma 100-400's, which is otherwise a stellar lens. Abbot shows that Tamron's new 100-400 is just as incredibly sharp as the Sigma. Does its VR work any better? Will I want to sell the 200-500 again and buy the Tamron? Weight-wise, this is a no-brainer, so stay tuned...