Is the Canon EF 17-40mm f/4L USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 20mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Canon EF 17-40mm f/4L USM Lens set to 20mm f/5.6 sharper than the Tokina 16-28mm f/2.8 AT-X Pro FX Lens set to 28mm f/16.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.