Is the Sigma 8-16mm f/4.5-5.6 DC HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 8mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Sigma 8-16mm f/4.5-5.6 DC HSM Lens set to 8mm f/4.5 sharper than the Tokina 11-16mm f/2.8 AT-X Pro DX Lens set to 13mm f/3.2? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.