Yeah, I figured out early on that it was 3XK, and I felt the cops were a bit slow to catch on that the whole thing was orchestrated to lead them along a path and that any given clue they were handed should be mistrusted. I mean, the recovered photo from the first victim led them right to the second victim, the connection they found led them to the plastic surgeon, her testimony about when Santos left her office led to the traffic photo that connected to William Mapother's character, etc.
And that was the reason for the tattoo. It was part of the manipulation. No, it wasn't needed for the imposture and the theft, but that's exactly why it was done: to divert them from thinking that there was a practical reason for the impostures and rattle them into thinking it was something more personal and creepy, something directly targeted at them. Classic misdirection. That's probably also why the killer chose Lainie and Esposito as the targets -- because that seeded the false trail that it had something to do with their relationship. (Although I guess it could've been just that they were the only ones the killer was able to find doubles for. I was starting to think, "How many of these guys have doppelgangers working in the sex industry?")
What's interesting about the ending is that it's ambiguous whether Tyson is actually alive or if the plastic surgeon is merely taking over the 3XK mantle now that he's gone. Taking his police records could itself be misdirection to make them think he's still alive.