Anyone on the IMDB Cast list could be RJ, and there's still probably some names not on the cast list for episode 8. I wouldn't rush to quickly to assume Mashburn is RJ.  I think a more likely scenario is that Jane calls on Mashburn for help. Seems more logical than him being RJ. Mashburn doesn't fit the physical descriptions of RJ (does not have notable posture or straight hair) and he was not on the list of 7 (unless Heller was lying about the list--seems unlikely).

GOing with my, gut, if the kill scene is Jane straddling RJ and killing him, I just can't picture Jane having that moment with Partridge, Bertram, or Kirkland. I could picture him having that eye to eye intense moment with Hafner. I can maybe picture it with McAllister. I really doubt RJ is Reed, but I could even picture him locking eyes in that scene.

Because it implies a real physical moment, I dont see Jane having that scene with Stiles either. No proof, just my gut.