I have shot with the tamron 150-600 g2 for about 2 years.
Great lens. Easy to hand hold. My copy is sharp- and I print large. I do not shoot brick walls so never tested the corners, but I had no complaints about sharpness.
It is a little slow to focus- but once locked on does well with birds in flight. Most of my images posted here this last year were shot with this lens.
It is too slow to track a bird coming straight at you- which my nikon long primes can do with ease. They also cost 5-10x as much. No free lunch.
Football season just started, so I am shooting games with the nikon 400mm 2.8. Too heavy for me to handhold- but wow what a lens. Puts the tamron to shame, but it should. It is sharper, focuses faster, better contrast, better color, better everything. It also cost almost 10x as much. Different tool for different needs. Is the tamron perfect? Nope. But when I am out handholding a lens in good light all day, the tamron is the lens I will pull out of the bag.
I have not shot enough with the sigma to have a useful opinion, I bought the tamron.
Gary