From a recent interview:

"I was surprised, and had I been Red John, I would have been equally surprised," she said.Robin, who plays Senior Special Agent Teresa Lisbon on the show, admitted she was kept in the dark until the episode.

To me, this tells me that (assuming the list of 7 is genuine) RJ is somebody who is not a usual suspect or a creepy or 'bad' guy. It seems like Bertrum fits somebody who would suprise both Lisbon and the actor playing Bertrum. Stiles would surprise neither. Haffner would surprise neither. Mcallister neither. (Remember, Robin is referring to her shock during the actual episode of the reveal, so given the first 5 episodes, Haffner or Sherrif or even Smith would come a no surprise to her). It must be either Bertrum, or perhaps Partridge (which would certainly surprise both of them!).

Or perhaps Robin is just being coy to the media, which is on second thoughts more likely. What do you think?