The problem is, every single theory I read, I can't get away from the fact that the sniper, as much as he was trained on John, would have been certain to look at Sherlock jumping, given that a) wouldn't you be slightly distracted and b) surely he wanted to be certain that he'd died in order to know whether to carry out the order? Even if he hadn't seen, Sherlock knew there was a hit man watching John, doesn't it seem like a huge risk to take?