I think I can't contribute to finding the RJ's identity but I just thought of some back-plot which may sound like fantasy but still I can't help but writing it for you guys:
Have you ever read the book "The Watchers by Dean R. Koontz" where it tells about a wicked experiment on 2 dogs in order to create thinking/calculating animals but one of them turns out to be a very smart monster whereas the other one stays a dog but super smart as well. And the whole story is about them trying to fight and trap (and naturally kill) each other.
Or remember the movie in which Schwarzenegger and De Vito were playing twins of same mother but a combination of fathers (which makes them practically sons of a bitch though :) and it was also some kind of an experiment to create a super human, namely perfect in any aspect but the two turned out to be far different from each other.
When we look at the concept "fearful symmetry", given the fact that PJ and RJ are very much alike except RJ has chosen a career in serialized crime which can be named as "fearful". So, them being actually siblings is not that a bad idea. After all, the same medicine may not have exactly the same effect on two people even if they're from the same mother. There might be some side effects and RJ seems to be suffering from those.
This would at least explain the love and envy of RJ towards Jane as RJ long ago figured out that they are the fruits of the same experiment. So, while being able to resist the idea of beating Jane in mind games, he may also feel the warmth of having him as the only family he has on this earth. But luring him by killing his wife and daughter was a mistake and he knows that deep inside. And he wants to believe that PJ will have hard times but finally forgive him (or be willing to accept his existence) when they meet. But obviously he's wrong and he gets killed. Maybe this is why the RJ killing scene is so much dramatic and sad which may make people cry (according to previews and spoilers). I mean you find out that you have a brother alive (that you never was aware of), yet you must kill him.
If this theory of mine comes to be true, let me tell you another thing. Remember when Visualize was running a barn with some sort of experimental approach in order to increase the productivity? Why not they should have run a similar experiment on people? Maybe the farm was a main clue about what's really going on. And do you remember when Stiles said to Jane that "I know a lot more about YOU and RED JOHN that you could ever imagine"? He didn't spare Jane in his statement. The question was Red John but reply was BOTH of them as if they are parts of the same thing.
Patrick was planted in a family with Carnival, and Red John was raised in somewhere else under different conditions. Maybe Stiles did not expect that one of them would figure out the other and try to communicate and mess everything up. Since both are his test subjects, they were vitally important to him. He can't give up any of them. So, he can't expose RJ's identity to PJ but he has nothing to do with RJ figuring it out. It's out of his hands now. He seems both proud and sad about the combat taking place between the two but he can't help watching it either.
Who may be RJ if this is the case if we stick to the list??? Kirkland, Partridge or Haffner. Because the ages must match between RJ and PJ if they will be brothers.
For the ones who're curious; No, I don't smoke anything out of ordinary (filtered camels only) so I can't send you the same so you can get as high as I am :))))
