Is the Sigma 105mm f/2.8 EX DG OS HSM Macro Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 105mm f/3.2 test image. Observe the contrast and resolution of these results.
Is the Sigma 105mm f/2.8 EX DG OS HSM Macro Lens set to 105mm f/3.2 sharper than the Sigma 105mm f/2.8 EX DG Macro Lens set to 105mm f/3.2? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.