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 100mm f/5.0 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 100mm f/5.0 sharper than the Canon EF 70-200mm f/4L USM Lens set to 280mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.