Is the Sony FE 300mm F2.8 GM OSS Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 420mm f/4.0 test image. Observe the contrast and resolution of these results.
Is the Sony FE 300mm F2.8 GM OSS Lens set to 420mm f/4.0 sharper than the Sony FE 400mm F2.8 GM OSS Lens set to 400mm f/4.0? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.