To prevent abuse of Riolu's Prankster Copycat Roar strategy, Copycat was nerfed such that if used by a Prankster using a negative priority move, it no longer had priority. Did Mirror Move receive the same nerf?
http://replay.pokemonshowdown.com/pokemondatabase-customgame-12427
As shown, Mirror Move does bypass priority, but you can only abuse it for one turn, as Mirror Move cancels out the minute that the opponent gets switched out. Mirror Move works only on the Move The Target used last, not the user, so Whirlwind+Mirror Move will not work.