Is the Canon EF 70-300mm f/4-5.6L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured mm f/ test image. Observe the contrast and resolution of these results.
Is the Canon EF 70-300mm f/4-5.6L IS USM Lens set to mm f/ sharper than the Sigma 120-300mm f/2.8 DG OS HSM Sports Lens set to 200mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.