Have you considered going down to maybe a 12inch sub so you can run a properly sized ported enclosure? If done right a ported box can get just as tight and punchy as a sealed box. You just need the right woofer and an enclosure designed for your music taste, listening habits, and in car acoustics. It sounds like more of a pain in the ass than it is I swear.
A whole world of options open up for you when you port an enclosure, you can adjust box volume and vent area to account for power used and cone control as well as what notes you'd like to be more prevalent. It really is a superior design.
I can guarantee that given the same power a 12 in a well designed and implemented vented enclosure will deliver output and SQ performance exceeding that of a 15 or more so an 18 in a sealed enclosure. ESPECIALLY one that runs on the small side. You could end up with a sealed 18 that has -3 down point in the high 30 to low 40hz range if your choking it in a sealed box that's too small.
better a smaller woofer in the right box than a huge woofer in the wrong box