He did a lot so far. Heather started off the season with an alliance, read Gwen's diary out loud, voted off Justin, sabotaged Trent out of the competition, formed an alliance with Duncan (for an episode), got Izzy voted off, betrayed Harold in the special, got Lindsay and Beth to do her things and did little in challenges.
Justin, on the other hand, formed an alliance with Beth and has full control of the Grips...already!, got Trent voted off (man, the dude should really get out of the way), forcing Owen, Lindsay and Beth to be his slaves but he is doing decently in the challenges.
Already Justin did almost half of what Heather did. We already know, as you said, that he's messing with Beth's feelings, which could result in another "Gwen's diary scene". In my opinion if he continues, he WILL top Heather but maybe he won't be hated as much by the viewing audiance and possibly the other campers.