Is the Canon EF 400mm f/5.6L USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 560mm f/9.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 400mm f/5.6L USM Lens set to 560mm f/9.0 sharper than the Sigma 120-300mm f/2.8 EX DG OS HSM Lens set to 420mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.