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 16mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Canon EF 16-35mm f/4L IS USM Lens set to 16mm f/4.5 sharper than the Sigma 18-35mm f/1.8 DC HSM Art Lens set to 24mm f/2.8? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.