Installed this catback system several days ago. It has more a a lower sounding rumble compared to others I have heard. Its not so loud but you always feel a nice little rumble at most speeds. Not sure if they tuned it so perfectly for my needs, but I may be just lucky that it because relatively quiet at around 3750-4200RPM on the highway which is approx 120km/h so it doesn't tire you out or become annoying. It complements the sound of the Mazdaspeed intake (AEM) because it carries the sport low rumble sound until about 4500RPM and then the intake joins in at about the same low frequency after that so it sounds very well tuned together.

As for additional power, I am not sure. I assumed no additional power when I bought it but my butt dyno tells me it may have given the car a little more responsiveness and the intake seems to breath more easily than before or sucks in more air.

Overall, I would strongly recommend this exhaust system if you want a nice and noticeable exhaust note but not one that is too loud. Very satisfied with the purchase and the burn titanium tips look amazing. Construction and materials used are very high quality with some adjustability to line up the exhaust pipes almost perfectly.