Is the Sigma 18-300mm f/3.5-6.3 DC Macro OS HSM C Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 135mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Sigma 18-300mm f/3.5-6.3 DC Macro OS HSM C Lens set to 135mm f/6.3 sharper than the Tamron 16-300mm f/3.5-6.3 Di II VC PZD Macro Lens set to 300mm f/7.1? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.