After fine tuning both lenses with LensAlign, I decided to compare the resulting images side by side just to see which lens was sharpest wide open. Several sample images were taken using both lenses after they were fine tuned and only the best image was kept for both. These samples represent the middle portion only (where the AF was pointing on the target). The 300mm f/4 PF and 500mm f/5.6 PF were tested with the same camera settings (Nikon D500), same lighting, on a tripod with remote shutter release, 2 second exposure delay and Markins long lens support system to make sure possible vibration would not influence results. Unfortunately I could not test the 500mm PF at the required distance from target because of limited indoor space therefore the resulting image taken with that lens appear slightly larger than that of the 300mm PF.

The two images appear very similar to me. I believe that at the same working distance from target, the 300mm PF image would be just about equal in sharpness. The 500mm PF sample does appear slightly sharper/more contrasty to me but again, distance from lens to target was not quite the same.

Although this comparison was only done for fun, It can give a general idea of how close both lenses are in sharpness wide open at the center focus area.