Is the Sigma 120-300mm f/2.8 DG OS HSM Sports Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 250mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Sigma 120-300mm f/2.8 DG OS HSM Sports Lens set to 250mm f/3.2 sharper than the Canon EF 600mm f/4L IS II USM Lens set to 600mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.