Well, I am not exactly an expert in the codebase, having been working with it for... two weeks. :) But! As far as I can see, there is no method for "tell me all the content filters I've put so-and-so into". (I am sort of hoping I'm just not looking for it in the right place, though.)

That said, I don't think the edit subscription page is inefficient, because it doesn't actually let you look up all the content filters that you've put a user into. It lets you pick a single filter and see who is in it and who isn't, which is exactly in line with the backend method that, given a content filter id, looks up which users are in it.