Is the Tokina 11-16mm f/2.8 AT-X Pro DX Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 12mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Tokina 11-16mm f/2.8 AT-X Pro DX Lens set to 12mm f/8.0 sharper than the Sigma 10-20mm f/4-5.6 EX DC HSM Lens set to 12mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.