Well, I can't give an actual db rating, I can tell you that it is, IMHO, well worth the money. I can't imagine one in a slot port box though
I will build one up for mine this spring / summer.
I can tell you also that it is louder then a pair of Rockford Fosgate HE2 12" subs on the same amp, also louder then a single 12" Power Acoustik Fubr sub in a huge box. I also have had a pair of Directed 15" subs in this car that this Alpine blows away.
About the only system I have had that was louder doesn't really count because it was in a *-10 Blazer, but that was a Directed 600d.1 ( same amp ) and a pair of Rockford Fosgate HE 15" subs..... With all the room inside that Blazer, the sound was just ridiculous. I could hardly drive down the road with it up because it actually distorted my vision