As far as I know, it's been confirmed by Heller that it is indeed one of the seven that was mentioned in the end.
I didn't even remember XB's episode, but I'm very confident that he'd be able to pull off RJ's psychotic charisma, so the whateveritscalledTVtrope where the actor spoils the plot by being there is very much in effect.
Brett seems like a herring, and the way he talks about RJ vs. Jane doesn't lead me to believe that he really is RJ. Granted, I suspected him since the first time I noticed him in Season 2's finale, but lately he seems to be more of a closet-fan of RJ and doesn't think too highly of Jane, whereas RJ does.
One of the main reasons I suspect Bertram is that he seems very inept at dealing with people the right way and asks for Jane's help in the poker-game. RJ would be able to pretend he's a nobody easily, as he's as proficient in the arts of mentalism as Jane. So the same could be said for Brett.
Apparently, Reede's name is an anagram for something like... "Theme is red" and "The red is me", so there's that, but I'd be really disappointed if he'd turn out to be him. Same with Bret (Malcolm). He has a strong character as is, so flipping everything about that character and making him the main antagonist feels off. And a waste.
I don't even think Kirkland is evil, despite his actions throughout the latest season. Smells of herring. He has some agenda for hunting RJ the way he does, but my guess is that he's one of the good guys.
As for Haffner... I really don't have anything to say about him. Except... something that came to mind just now. In his first appearance he asks Cho a few questions about intelligence regarding him and Jane. It's subtle and natural enough, but it's always stuck with me. Not in any suspicious sense, but... well, until now.
I have no concrete definite answer at the moment; I'm just happy if I figure it out before you're supposed to.
Oh. And it's been confirmed that Jane's NOT RJ. But you can be damn sure that RJ has hypnotized him.