Is the Canon EF 100-300mm f/4.5-5.6 USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 135mm f/5.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 100-300mm f/4.5-5.6 USM Lens set to 135mm f/5.0 sharper than the Canon EF 75-300mm f/4-5.6 III Lens set to 200mm f/5.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.