Is the Canon RF 100-400mm F5.6-8 IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 400mm f/9.0 test image. Observe the contrast and resolution of these results.
Is the Canon RF 100-400mm F5.6-8 IS USM Lens set to 400mm f/9.0 sharper than the Canon RF 100-400mm F5.6-8 IS USM Lens set to 135mm f/7.1? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.