Is the Canon EF 70-300mm f/4-5.6L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 300mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Canon EF 70-300mm f/4-5.6L IS USM Lens set to 300mm f/5.6 sharper than the Canon EF 70-200mm f/2.8L IS II USM Lens set to 280mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.