I really haven't done much testing of either 80-400 (D or G) at 300mm, and most of my 300F4 (AF-S model) has been done with a Nikon or Kenko 1.4TC.
I would have to say that the 300F4 (AF-S is the only one I have experience with) is likely to be sharper than the 80-400AF-D at any aperture, but 300mm is certainly much sharper than 400mm in the zoom.
It may be that the 80-400AF-D is 'sharp enough' at 300mm (though I have suspicions it will be noticeably worse wide open). The 300F4 is 'sharp enough' at any aperture.
If I can motivate myself to do it, I could take the 200-500, the 300f4 and both 80-400's out for a comparison test, but I don't see a lot of use to doing so. I just didn't get the zooms to shoot them at 300mm, and sharpness differences there don't mean much to me.
It might be better to compare the zooms against the prime in other areas where the prime has the advantage. For instance, shooting hummingbirds. I can get really nice shots with all of the zooms, but the close-focusing ability of the 300f4 means I could potentially get more pixels on the subject. I noticed when shooting the 24-70, 70-200 and 200-500 at the zoo recently that several times I had to swap out the 200-500 simply because I was too close to the subject to focus, regardless of focal length. There is a whole range of subjects that are better shot with a 300F4 than these zooms because of that.
