Is the Sigma 150-600mm 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 200mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 150-600mm f/5-6.3 DG OS HSM C Lens set to 200mm f/8.0 sharper than the Nikon 200-500mm f/5.6E AF-S VR Lens set to 500mm f/6.3? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.