I don't mean to say that you can't deadlift with hex-plates; there are guys at my gym who pull 495lb for singles with hexplates.
But I'm willing to bet that those guys can do a lot more than just 495lb on the deadlift if they had circle plates. After all, these guys also squat 405lb+ for reps. It doesn't make a lot of sense that they can only pull 495lb. I'm willing to believe that they're intentionally limiting themselves on the deadlift.