Is the Canon EF 300mm f/2.8L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 600mm f/5.6 test image. Observe the contrast and resolution of these results.
Is the Canon EF 300mm f/2.8L IS USM Lens set to 600mm f/5.6 sharper than the Canon EF 300mm f/2.8L IS USM Lens set to 600mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.