Is the Canon EF 70-300mm f/4-5.6 IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 70mm f/8.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 70-300mm f/4-5.6 IS USM Lens set to 70mm f/8.0 sharper than the Tamron 70-300mm f/4-5.6 SP Di VC USD Lens set to 200mm f/5.6? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.