I've read the main benefit is it helps keep your HP #'s consistent as it keeps the air cooler, rather than giving you more HP. Obviously as temp goes up, the car isn't going to run as good.
IMO, it's worthless on a DD, especially if you don't have many other mods. Just another part to throw money at. Unless you're doing some mild racing where this would actually come in handy, I just don't see a reason to get it.