There's really no way of telling what it'll be hitting db wise. There are just too many variables (car, install, charging system, etc.) As for competitors there's the SS Rl-P and the W6 like you were sayin, but all i hear is good things about the arsenal so i'd say go w/ them. I dont't think you'll find a noticeably better sub for the price.
Also, AA recomends a sealed box for those subs. I have 2 15's and I will be putting those in a sealed box. I am growing out of the pure SPL phase. Everyone around here runs audiobahns and some sonys so I am not worried about being LOUD. Just want clear sound. AA is definatly the way to go though. Too bad you missed out on the pre order.
i just got an arsenal 12 and its in a ported box.... its soooo good... i have a friend that has 2 12" cvr's and my aa is SO close in spl, but it has infinitely better sq... just fyi...lol