When I first got my Sigma 150-600 Sport I was disappointed too.
But after looking at my pictures and talking to my mentor it came down to two items.
1) Method used to hold the lens. After all a 600mm, 6 lbs lens needs to be handled much differently then the Canon 100-400mm lens. I have both of these lenses.
2). Then there is the air quality conditions and camera setup.
I am now lifting 10 lbs weights at least 3 times a day. I went out yesterday taking hand held shots at 600mm and good air quality.
The results were very sharp pictures. Now I am happy with this lens.