Is the Sony FE 600mm F4 GM OSS Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 840mm f/6.3 test image. Observe the contrast and resolution of these results.
Is the Sony FE 600mm F4 GM OSS Lens set to 840mm f/6.3 sharper than the Sigma 60-600mm F4.5-6.3 DG DN OS Sports Lens set to 400mm f/7.1? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.