. My capital ships come out okay, but my fighters always look like they're going twenty miles an hour, even with moving starfields.
Have you considered adding some aberrant motion to the ships? When a fastmoving car swerves a little, there's some back&forth fishtailing, and likewise if you're doing course corrections and such, you'd have the space equivalent.
Having the camera struggle to keep the ship framed (a la THE RIGHT STUFF, which is a good example of selling speed with rubber bands & tape) also might give the speed illusion a bit more oomph.