Is the Canon EF 28-300mm f/3.5-5.6L IS USM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 70mm f/5.0 test image. Observe the contrast and resolution of these results.
Is the Canon EF 28-300mm f/3.5-5.6L IS USM Lens set to 70mm f/5.0 sharper than the Tamron 18-270mm f/3.5-6.3 Di II VC LD Lens set to 50mm f/8.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.