Is the Sigma 14-24mm f/2.8 DG HSM Art Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 16mm f/11.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 14-24mm f/2.8 DG HSM Art Lens set to 16mm f/11.0 sharper than the Nikon 14-24mm f/2.8G AF-S Nikkor Lens set to 18mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.