Normally you want the sub to get close to mechanical efficiency with either an efficient enclosure or a lot of power on tap. Sure you can throw 1000 watts to a pair of xfls in a 5 cubic feet gross box in his trunk and they will make some sound. However, Will it sound impressive? FK no(to basshead standards), neither proper space nor enough power to fully utilize the sub's potential and you will feel that lack of bass when the sub hits vs a sub that reaches mechanical efficiency via power or efficient box.
Its not really a matter of underpowering as said before, its more like not getting the most out of your sub(potential wise)/money/time invested. If you really want the max out of those xfls, you'll either need to step down in size to get a really efficient box in or upgrade electricals and really drive them hard with clean power. At least 2k to 2.5k watts rms of power from a good reputable amp if wired to 1 ohm.