ya HX2's and L7's are the only mainstream decent subsi could think of...
beyond audio inhuman... best bang for the buck IMHO...have you ever heard a Beyond Audio Inhuman 15" with an Earthquake D2 Shredder hooked up to it in a slot ported 7.5 cu.ft. box tuned at 50hz? absolutely amazing... and yes i have heard W7, in a car, although it was only powered by a JL1000.1, it sounded amazing too, but like i said... $$$
why not go Inhuman? cheaper than W7

both are SQ subs but Inhuman puts out higher SPL...
in terms of sub boxes and how many you should get, go with one sub in a about a 4.5 cu.ft. box, slot ported... if you're gonna compete, it's the best way to go, since very few people hit over 140db in the single sub class (12" or less, dunno about 13").. with a W7 powered with 1200W, you'll prolly hit around 142db if you do it right...
i have an Orion HCCA 12" which will have 1000W RMS goin into it... it's a beefy sub and the box that i made is close to 4.5 cu.ft.
dimensions for the box is 30" wide x 24" long x 11" high (nearest hatch) and angles up to 14" high (near backseats)... made outta 3/4" mdf, and it's a solid mofo...
dunno but i think my Orion is as close as you're gonna get to a W7... i'll let you know how it sounds when it's all installed in a couple weeks...