Is the Sigma 18-250mm f/3.5-6.3 DC OS HSM IF Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 80mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-250mm f/3.5-6.3 DC OS HSM IF Lens set to 80mm f/8.0 sharper than the Tamron 18-270mm f/3.5-6.3 Di II VC LD Lens set to 70mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.