Is the Sigma 17-70mm f/2.8-4 DC Macro OS Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 70mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Sigma 17-70mm f/2.8-4 DC Macro OS Lens set to 70mm f/4.5 sharper than the Sigma 24-70mm f/2.8 EX DG Lens set to 70mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.