Is the Canon RF 600mm F4 L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 1200mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Canon RF 600mm F4 L IS USM Lens set to 1200mm f/8.0 sharper than the Canon EF 600mm f/4L IS III USM Lens set to 1200mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.