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