[lobster_johnson]

But if you're maintaining a separate ts_vector column, which you then index, you're creating the exact same amount of index data.

Unless you're saying that you would populate the field only on some rows and not all of them, and control this from the app. But you could do that with an expression index, too, assuming the rule is a simple, pure function:

  CREATE INDEX index_posts_on_body
  ON posts (to_tsvector(body, 'english')) 
  WHERE published = true;

or similar.