I rewatched that season 3 finale, and except for the fact that such fake RJ is very dead, he had a voice very much like RJ´s voice in this Season 6 episode 1.
I may change my mind in the next minute, but I´ve watched again S6E1, and noted that when Lisbon finds Partridge dead, he can barely say tiger, tiger, and apparently dies, but check this out, Lisbon was pulled back by who ? the real RJ I think-
Patrick calls Lisbon, but RJ answers Lisbon´s phone.
That being said, I think that when Lisbon got to that place, there was Partridge (we assume he´s dead now), but there was somebody else there, I´d say the real RJ.
We´ll find out soon, who is the real RJ