That's kind of the point though - I had no idea this feature existed until they started pushing it to me. I think it's too much to expect users to opt-in to things. How would I have known to opt-in to this? Yet I am now much happier because they improved the UX for me.
I think opt-out makes sense though, for users who don't a feature.
If you put it on a single, easily recognizable, easily accessible page (all new features, etc.), then many users will find it. You can even give a notification when a new feature is added (as long as kept to a reasonable amount).
Features that will go viral are worth adding for all, features that did not can be kept there.
You overestimate what people will understand from a technical description in a configuration option. Even if they were experimentally inclined, which most people are not, they probably wouldn't even notice the effect, particularly if not a new user. Twitter is a black box for most users.
Twitter very likely did market research on its effectiveness for the target market they were trying to please. Probably that's just not you.
I think opt-out makes sense though, for users who don't a feature.