Is the Sigma 120-400mm f/4.5-5.6 DG OS HSM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 560mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Sigma 120-400mm f/4.5-5.6 DG OS HSM Lens set to 560mm f/8.0 sharper than the Tamron 150-600mm f/5-6.3 Di VC USD Lens set to 400mm f/11.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.