I don't think red john is one specific person. Its def Bob Kirkland with Volker and some others who would know such as Kirstina Frye, Lorelei Martins. In season 5 episode 8, Bob comes in telling Lisbon that he wants her to let go of the Volker story. The way Volker kills Amanda (secretary) is the same harmful and disgusting way Red John kills his victims (also by using someone). Also, when Bob walks in he incorrectly pronounces Lisbon. This is a very minor supporting example, yet it is significant because Im pretty sure everyone knows who Lisbon is. His high voice is also shown 2 episodes ago.

Also, what happened to Kristina Frye? She suddenly dissapeared? She definately has something to do with it.