As it's quiet here so I'm posting some observations. I've been using the Tamron 150-600mm zoom for about a month now (second copy, first de-centered) and am very pleased with it, it's easily hand holdable and for the most part a sharp lens, holding up very well till 500mm where it's very good at F-8-11. At 600mm the inevitable happens and sharpness drops a little, but with a little sharpening very decent results remain. The VC is a great bonus and works very well although it has a slight tendence to hang on and let go, which causes a little jerking if you have drunk too much coffee. The AF needs no AFMA and seems spot on.
I've noticed however, on looking at images @ 500-600mm at 100%, most noticeable in the fine detail of grass, very small patches of localized blurring occurring in the frame. I've also noticed this with the Bigma maybe to a worse effect.
I just wondered if this is caused by the lack of perfect smoothness of the lens elements which causes these localized patches of softening and it does suffer from onion ring specular highlights, I don't think it's of any real concern given 100% magnification and am not suggesting a fault with the lens, just observing this effect!
The accompanying image shows this small soft patch (there seem to be three or four of them across the frame) in an otherwise decently sharp 600mm 100% crop. I've seen this several times always on lenses of 400mm or longer.
Sorry I should have highlighted the soft area but it is in the middle of the clip!