The Sigma 150-600mm C has had a really varied set of reviews and MTF measurements published, indicative of inconsistent quality control. I tried one last year from my local shop and it was about the same at 600mm f/6.3 as my 100-400mm II + 1.4xTC at 560mm. Yesterday, the local shop had a new one in and I borrowed it for some chart tests, and it bowled me over. OK, the OS (IS) is all over the place compared with Canon, but it was far sharper at 600mm and f/6.3 than my 100-400mm II at 560mm, and also sharper when both at 400mm. It was even sharper and with more consistent focus than my 300mm f/2.8 II + 2xTC at 600mm. It was clearly a high end outlier and I had to buy it (traded one of my 100-400 IIs with cash back). It then out performed my Canons in IQ shots of goldfinches on my feeder. This is one of those lenses you have to try out first t get a good one, and this one is tops. Here is one of my shots from yesterday. Tomorrow, it will be tested thoroughly. There is no sharpening applied to this shot, just RAW converted denoised with DxO (iso1250, 1/200s, f/6.3 and 5DS R).