Is the Canon EF 16-35mm f/4L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 20mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 16-35mm f/4L IS USM Lens set to 20mm f/8.0 sharper than the Sigma 17-50mm f/2.8 EX DC OS HSM Lens set to 21mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.