If you're coming from a JL 500/1 and a single 12" Arc sub in a sealed enclosure you sure as hell don't need two 15's to get LOUD in comparison. Also as already mentioned a pair of 15's in a trunk isn't going to be easy to build and will likely require the enclosure be built in the trunk. It doesn't sound to me like you have the experience necessary to take on a build of the proportions you're aspiring to and have it come out anywhere near optimal. That being said, I think you would be VERY happy with a pair of 12" Fi X series subs, SSA GCON's or even some Fi SSD's in a properly designed and built ported enclosure on a 1500 watt amp. It will be less expensive for the equipment, will be easier to build and implement, and won't require the sheer support in the electrical department and sound deadening that a 3500 or 5500 watt system would require. It will cost you less money, gain you some experience with doing the building and installation yourself, and still get WAY louder than what you're currently using. That's just my .02