There is fairly strong evidence to suggest that PRP can speed the recovery of damaged tissue. There is only one study (to my knowledge) about the penis. They damaged the penile nerves of rats. They injected PRP and found that the PRP group recovered faster.
(sorry if that link has been posted already, I have not had time to read this thread)
PRP can almost certainly induce additional growth in healthy tissue.
My problem with Dr Runels is the extremely poor quality of his site and his conflict with the authorities.
I doubt that a single shot of PRP would do anything for enlargement. The treatment probably does have potential if multiple injections were administered over the course of a couple of months.
Anyone who is interested in this avenue could actually buy the centrifuge and other materials online. They could give themselves as many injections as they wanted. That would probably work out cheaper than a visit to Dr Runels.