How sharp should a filleting knife be?
It should be slightly duller than a fresh razor blade.  Skill can make up for lack of sharpness, but if you don't have the skills you'll mash the fish more than slice.  This is why instructors and surgeons can get away with dull knives; they've got years of experience cutting flesh.  

Traditionally, fillet and boning knives are sharpened to a shallower angle than general-use chef knives, to allow a keener edge. The only knife in a cook's arsenal that should be sharper is the slicer, which sees rarer use, and needs to be super-sharp for thin slices.