Even if it turned out to be faked, this is a truly amazing piece of acting an an absolutely heart-wrenching scene. We've just seen Moriarty shoot himself in the head, apparently closing out any shot Sherlock had of escaping. The first time you watch it, Cumberbatch's acting is so painfully good you absolutely believe what's happening is really Sherlock's ending. It's interesting, upon re-watch, that Sherlock tried to convince John he was a fake. He could have come up with another explanation for his suicide quite easily--he didn't have to use that one, to attempt to disengage John. He tried to convince John he was a fake and a traitor, right before convincing him he had committed suicide. It seems like this was a (probably terrible) attempt to make things better for John after the fake suicide, to push John away so that he may feel less guilty. Which was nice of Sherlock, even if the entire plan to trick John was horrible. What comes across most strongly in this scene is the powerful connection between these two men, communicating across a literal vast gulf, one always knowing more than the other. Even though there's that distance, and that intellectual disconnect, Sherlock and John's emotional connection comes through loud and clear.