Is the Canon RF 600mm F11 IS STM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 840mm f/16.0 test image. Observe the contrast and resolution of these results.
Is the Canon RF 600mm F11 IS STM Lens set to 840mm f/16.0 sharper than the Sigma 100-400mm f/5-6.3 DG OS HSM C Lens set to 400mm f/7.1? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.