Strictly theoretically, anything that can reflect sound back to (and, depending on the construction, through) the driver will cause an apparent high-end rise. It's worth it to do some experimenting, since many people feel Grados are bright. Keep in mind that the Grado driver already has a fairly dense damping pad over its vents, so chances are that any changes you'll get will be slight.
I agree that removing the grille is likely to produce a larger change than removing the button. The button, by the way, is held on by hot glue, and pulling it off may or may not tear a small hole in the grille-- no damage done soundwise, although of course the aesthetics (?) suffer and small creatures can get in to make nests.
Now, if you place discs of paper over the grille and listen, you will probably hear the highs start to increase, depending on how many paper discs you use and how dense the paper is. If you think Grados tend toward the bright, you'll probably prefer them grille-less, although again, the change is likely to be slight.
While we're on the subject, it would be interesting to hear how much the bass would be affected (theoretically it would smooth and increase the low bass) by extending the rear cup outward with tubing or pipe.
