Is the Canon RF 800mm F5.6 L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 1120mm f/16.0 test image. Observe the contrast and resolution of these results.
Is the Canon RF 800mm F5.6 L IS USM Lens set to 1120mm f/16.0 sharper than the Canon RF 800mm F5.6 L IS USM Lens set to 1120mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.