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 70mm f/4.5 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 70mm f/4.5 sharper than the Sigma 150-500mm f/5-6.3 DG OS HSM Lens set to 400mm f/6.3? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.