Is the Canon RF 15-35mm F2.8 L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 20mm f/2.8 test image. Observe the contrast and resolution of these results.
Is the Canon RF 15-35mm F2.8 L IS USM Lens set to 20mm f/2.8 sharper than the Canon EF 16-35mm f/2.8L III USM Lens set to 20mm f/2.8? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.