I'm sorry. I am not a New Yorker, and even though I was aware it was Central Park, I just got that from "New York" and "Park". The shot where they are sitting on a rock? Thats the easiest composite ever. The shot where the Doctor sits down? I don't remember seeing that in the episode, although I guess it takes place after Rory is first taken? It certainly never happened at the end.
The shots where Rory walks through the city are something much harder to do through a composite and certainly wouldn't have the same feel. It reminds me of the City of Death - it just feels different when filming in the actual city in question.
Plus, you have no idea how much David Tennant was bitching about how he couldn't go to New York for the Dalek episode.