I doubt most know Harper or Cameron. Most would have known Blair, even Brown after so long, but Cameron hasn't really stuck out. Neither has Harper, despite being there forever. And I bet most Americans still think Fidel Castro is in charge. Or know Raul simply as "Fidel Castro's brother" instead of his actual name.
I think a lot of us overestimate the number of people that knows anything about the politics of foreign countries (an easy mistake to make if you hang around at a place like this). A 10 year old survey showed that only 6%
of Americans knew the name of the Canadian PM. And even though this was when Blair was at the height of his fame (and particularly in the US), only 51% could identify the British Prime Minister. In 2000 only 22% knew about Blair - I would be surprised if Cameron's name recognition is any higher than that.