Is the Canon EF 400mm f/2.8L IS III USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 400mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 400mm f/2.8L IS III USM Lens set to 400mm f/4.0 sharper than the Canon EF 200-400mm f/4L IS USM Extender 1.4x Lens set to 800mm f/16.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.