Is the Canon EF 200-400mm f/4L IS USM Extender 1.4x Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 560mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Canon EF 200-400mm f/4L IS USM Extender 1.4x Lens set to 560mm f/6.3 sharper than the Nikon 200-400mm f/4G AF-S VR II Nikkor Lens set to 680mm f/6.7? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.