First thing we know. Whoever Red John is, was there in the house of PIGEONS with Lisbon in S06E01, as he had to have painted the face on Lisbon. Partridge was there. We do not see him die, and we do not see anyone else in the room. Could it be that Partridge fakes his death so that Lisbon wouldn't suspect him when she wakes up (his plan all along).
http://www.youtube.com/watch?v=w4mhDdZPD-o

Second thing we know. Red John was part of the Blake Association (BA), he would have had the tatoo on his arm. Brett Partridges upper arm in the Cremation room was cut out. A bit suspicious? If it was a body substitute, BA wouldn't want the substitute to be identifiable from Partridges own body. Also, the act of cremation itself is a bit suspicious, why would they burn the body? So it cannot to identified? In the scene, no one checks to see his face! 
http://www.youtube.com/watch?v=hgWQ8AhfrGc
 
Third thing we know. "Faking your own death was easy. Brett Partridge was Blake Association. His job gave him access to the primary DNA database. He switched your DNA records with one of a body you had on ice" (S06E08) Partridge could have easily faked his own death, McAllister admits it himself, and so does Jane.
http://www.youtube.com/watch?v=ci-aR7jD9Yc

Lastly, the god damn voice. I know that Simon Baker does the voice for Red John, but is it not possible that he does the voice in the style of Red John's character?
http://www.youtube.com/watch?v=1TcWNyrWaZ0

IT IS POSSIBLE!