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 Tamron SP AF 17-35mm f/2.8-4 Di LD IF Lens set to 20mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.