I would bet that at 35 hz One of your L7's in the right box would be about 5-6 dbs louder than your setup now. And 35 hz is where u really feel the difference "to your ear" as well as on the mic.
Well honestly i dont know the optimum box for these subs. I built a box for two 15" L7s and we are pretty sure its too small and tuned to high. It is about 6 ft tuned to 38hz. IF you go with one, with 1000+ (probably near 1200) watts from that JL, with the parametric eq and all that, i would aim for about 3.5-4 feet at about 32hz. That would be decent. Ask someone who has had more expirience with the subs. Polecat?
Yeah it looks like you have enough room for a much much better system than what you are listneing to right now. Download a 30hz sine wave, play it in your car, and imagine that note being able to play 5 times louder with a new box. With only about 600 watts to each of those L7's in a 2.0ft sealed box, the low end is lacking.
If i was closer id build you a box for pretty cheap just to hear it. But your in Ohio im in WI. Get some plans and have a local place build you a nice box. Then try to sell your current box to some n00b. Dont let him hear your system tho. lol
With all the room you have in that Scion, try to get about 4.5 cu. ft. per sub tuned to about 32. hz if you want it to "sound" loud or tune it way higher if you want it to hit high SPL on the mic. But with a JL 1000/1 you wont win any SPL competitions because its rated 1000 at 4 ohms and 1000 at 1 ohm so people will have amps pushing 5000 watts at .5 or 1 ohm in the same class you would be in.