I have read this thread with interest as I have only been a Nikon owner for about a month. I bought the Sigma 150-600 Sport, mainly for aircraft photography. Bought the USB dock with so I could fine tune the lens. So far I have only adjusted the focusing speed. I'm pleased with the results not found the images soft at 600mm to the point adjustment is required. The lens achieves focus quickly and holds well on all aircraft types.
Here's a couple from last Saturday at an airshow in Scotland. The display was over the sea so the aircraft was some distance away.
For these images I used the lens on a Manfrotto tripod with Gimbal head.
Derek