There is something I just realized, they said Simon did those (at least some) Red John voices right? Now please imagine the upcoming scene when finally Patrick meets Red John face to face, let's say for example that Red John is, I don't know, Gale Bertram who has a deep voice, very different from that Simon-Red-John-voice on the phone calls.
Now the thing is will they just dub a face-to-face scene? That would be really stupid and nonsense, so my point...I think none of us is considering the voice itself as a huge clue as it seems to me, all that clues like ''suspect X is left/right handed and so he is Red John" or "someone said something in X episode that means he is Red John" I think those theories are unlikely but not impossible.

Back to my point, I assume that the pitched voice is his real voice, why? Easy. Remember that episode when Patrick meets a little girl who send a message to him? When Teresa and the team arrive, Patrick reveals that he hypnotized the girl because everyone who can identify Red John ends up dead, but before that he asked the girl how he was. AND if I recall right, she says something about his voice like "funny voice" something like that...and also the season 2 finale when they're face to face (or face to mask) and Red John has that voice just like that season 6x01 during the phone call, so I think it IS a relevant clue, therefore there are only two of the seven suspects that fits that voice, I mean, that can reach that voice easily, Bob Kirkland and Brett Partridge.

The show itself shows Bob as the more likely real Red John during season 5 so, a lot of movies with investigation involved, a lot of times the most obvious suspect is actually the real one.
About Partridge, he isn't a obvious suspect at first, but after checking websites like this one, there are a lot of plausible theories including him and so, he might be our guy and also I think he's the most likely Red John of all suspects.

PS: I think every theory here if it has a good point it is plausible, even if you don't agree with, I mean a lot of theories here including Bertram or Haffner I don't agree with, however they are plausible in some way. My point, don't downvote a theory if you just don't agree with it.