Is the Sigma 300-600mm F4 DG OS Sports Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 400mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 300-600mm F4 DG OS Sports Lens set to 400mm f/4.0 sharper than the Sony FE 400mm F2.8 GM OSS Lens set to 400mm f/2.8? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.