Is the Sigma 50-500mm f/4-6.3 EX DG HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 500mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Sigma 50-500mm f/4-6.3 EX DG HSM Lens set to 500mm f/6.3 sharper than the Canon EF 400mm f/2.8L IS USM Lens set to 400mm f/3.2? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.