Is the Sigma 100-400mm f/5-6.3 DG OS HSM C Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 135mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 100-400mm f/5-6.3 DG OS HSM C Lens set to 135mm f/8.0 sharper than the Canon EF 100-400mm f/4.5-5.6L IS USM Lens set to 300mm f/16.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.