Looking at Canon's own MTF charts for the bare 300, 500 and 600mm lenses (the mk2 versions) the 500mm appears to have the least drop off towards the corners (in sagittal resolution if I am reading the charts correctly), less than the 300 and 600mm. I realise Canon's MTF charts are theoretical, but based on this, how come the 300mm is sharpest? It appears to have the largest drop off of the three lenses.
300mm IS II: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_300mm_f_2_8l_is_ii_usm
500mm IS II: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_500mm_f_4l_is_ii_usm
600mm IS II: http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup/ef_600mm_f_4l_is_ii_usm
One of the main reasons I picked up the 500 II lens. Here is an example. I have another 500 mm prime that cannot get close to the edge sharpness of the 500 II. It really is amazing IMO.
This is on a 1.3 crop sensor. But still shows the power of the new lenses.