Is the Tamron SP AF 17-50mm f/2.8 XR Di II VC LD IF Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 17mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Tamron SP AF 17-50mm f/2.8 XR Di II VC LD IF Lens set to 17mm f/2.8 sharper than the Sigma 17-70mm f/2.8-4 DC Macro OS Lens set to 28mm f/3.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.