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 120mm f/8.0 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 120mm f/8.0 sharper than the Nikon 300mm f/2.8G AF-S VR Nikkor Lens set to 420mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.