Is the Canon RF 15-30mm F4.5-6.3 IS STM Lens sharp? This page visually answers that question. Shown above are 100% crops from a lab-captured 15mm f/4.5 test image. Observe the contrast and resolution of these results.
Is the Canon RF 15-30mm F4.5-6.3 IS STM Lens set to 15mm f/4.5 sharper than the Canon RF 15-35mm F2.8 L IS USM Lens set to 15mm f/2.8? The image quality comparison above also answers that question. Switch between the results to see the sharpness differences between these lenses.