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 250mm f/5.6 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 250mm f/5.6 sharper than the Nikon 200-400mm f/4G AF-S VR II Nikkor Lens set to 200mm f/4.5? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.