A lot of the newer subs have plenty of output even in small enclosures. I'm putting one these JBLs in the back of my BMW wagon. I'm using a sealed box, I like the tight fast bass they produce.
I'll also be using a JBL MS-8 signal processor. It does time alignment and EQ based on measurements made from microphones placed where the drivers head usually is. I can't wait to get it all installed.
Here is my Porsche install thread, it is relatively simple as I just needed something I could hear over the engine.
http://forums.pelicanparts.com/porsche-911-technical-forum/470332-911-sq-stereo-install-custom-sub-enclosure.html