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 800mm f/13.0 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 800mm f/13.0 sharper than the Tamron 150-600mm f/5-6.3 Di VC USD G2 Lens set to 840mm f/10.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.