The "Skyactiv-X thing" absolutely works and is available on Mazda 3s in Europe now. It doesn't work in the US because of our very stringent NOx emission rules (which, incidentally, is one of the reasons diesels are less popular here). I'm not aware of Mazda trying a Miller cycle engine outside of one trim level on the Millenia. I have heard that their experiments with that helped the development of their Skyactiv engines in general though. I'm also not aware of anyone else making Miller cycle engines currently. Perhaps you're thinking of Toyota's heavy use of the related but not the same Atkinson cycle engine.