That is an odd question, why would you doubt such a lens isn't sharp? Wh wouldpay that much money if it wasn't?
For a more specific comparison against the 400mm f/2.8 + 1.4xTC, the 600mm is sharper and will have measurably more reach and without any issues that TCs bring. The 400mm is tempting because it gives you the f/2.8 when needed but it is only worth it if you know you will be shooting large enough animals close enough to make use of 400mm frequently enough. If you are shooting birds then the 600mm is the logical choice. The 400mm is more aimed at sports than wildlife. if you cna get close enough to your subject to fill the frame with the 400mm then it will be slightly sharper, moreover a slightly different perspective gives a little more 3D look, and less haze. But that is big caveat.
The 500mm f/4.0 is actually by far the most popular super tele because it is a compromise of reach, weight, size and aperture - plus it is decently cheaper than the 400 and 600mm models. The latter 2 can have serious issues transporting in small regional planes whereas the 500mm will genuinely go anywhere.
