@Scott Murray. I've stayed out of this thraed for 2 reasons.
One ,is because I don't know anything about the new Tamron G2 150-600mm lens, and the other reason is that early on they were some bad copies of the Nikon 200-500mm lenses with focus problems. I don't know if this is still the case however.
My copy of the Nikon is sharp and has no focusing problems. I never fine tuned it to my D750.
It is as sharp at 500mm wide open as it is at other focal lengths and apertures.
Sometimes I use my Kenko 1.4 TC with it, but I rather not if I don't have to. Works great with slower moving wildlife but not very good for tracking fast BIF action.
If I was looking at telephoto today, I would be comparing this lens and the new Tamron. I'm glad I don't have to go through that again.
Yes i have heard that there have been Quality issue regarding the 200-500 if I do end up going for this lens I will definitely be testing it at the camera store prior to purchasing it.
Thank you for the response, just a question. Do you have any images with the 1.4TC and the 200-500? As I have that TC here somewhere aswell and would be keen to see what it is like.
I'm late to the discussion as well, but just throwing my $0.02 worth in (before it's not worth any less).
I've had an opportunity to shoot the Tamron 150-600 G1 on a few occasions. I now currently own the Nikon 200-500. From what I see of the G2 upgrades that Tamron is doing across it product lines, I suspect the 150-600 G2 will be even better than the original product, without question. That said, for me ... I think it's the fixed f/5.6 on the Nikon that really helps with the AF performance. More light gets in, so it's easier to track and focus. When I run a 1.4 TC on the 200-500, the AF performance is more like what I experienced with the 150-600. With the G2 lens still being an f/6.3 on the long end (they didn't improve the aperture across the zoom range, right?), I don't see it being any more snappy to auto-focus than the 200-500.
With the right light, the extra reach of the 150-600 is great, but the performance trade off ... at least with the G1 ... was too much for me.
I am holding off right now as may get a different lens.