If fitted one to my DM7. Definite, noticeable difference straight away - a touch smoother, much quieter. That said, not something you pick up that much when playing, but I needed a new bolt anyway.
I had one on my DM9, didnt reall notice much difference tbh. I would say that it would make a difference if your gun is new as the original bolt and orings arent worn in. If your gun is well worn in I cant see the billy wing making any difference.