James,
just saw a couple of early reviews of the new Tamron 150-600mm f/5-6.3 VC and it looks like I'll have to rent that guy once the rental places get in some stock and see how it compares. Looks like a lunger in terms of hand-holding but a lens with reasonably good IQ that goes out to 600 mm and has VC is something I need to check out more thoroughly. Look for a review update sometime soon.

Jim