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 28mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 17-40mm f/4L USM Lens set to 28mm f/4.0 sharper than the Tamron SP AF 17-50mm f/2.8 XR Di II LD IF Lens set to 35mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.