Is the Tamron 17-35mm f/2.8-4 Di OSD Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 35mm f/11.0 test image. Observe the contrast and resolution of these results.
Is the Tamron 17-35mm f/2.8-4 Di OSD Lens set to 35mm f/11.0 sharper than the Sigma 17-35mm f/2.8-4 EX DG HSM Lens set to 20mm f/3.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.