Is the Canon EF 100-400mm f/4.5-5.6L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 200mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 100-400mm f/4.5-5.6L IS USM Lens set to 200mm f/8.0 sharper than the Sigma 50-500mm f/4-6.3 EX DG HSM Lens set to 50mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.