Is the Tamron 15-30mm f/2.8 Di VC USD Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 30mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Tamron 15-30mm f/2.8 Di VC USD Lens set to 30mm f/2.8 sharper than the Sigma 20mm f/1.4 DG HSM Art Lens set to 20mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.