Is the Canon EF 24-70mm f/2.8L USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 24mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 24-70mm f/2.8L USM Lens set to 24mm f/8.0 sharper than the Sigma 24-70mm f/2.8 EX DG Lens set to 70mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.