Is the Canon RF 100-300mm F2.8 L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 420mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Canon RF 100-300mm F2.8 L IS USM Lens set to 420mm f/4.0 sharper than the Canon RF 100-300mm F2.8 L IS USM Lens set to 600mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.