The possibly gain some efficiency, but stick with what Mike (soundsplinter) has listed on his site. He knows these drivers better then anyone.

3 cu ft is kind of the happy medium. For a slightly flatter response (with a slightly quicker drop-off, Q of ~ 0.7), 2.1 cu ft net volume per RL-p15 is perfectly acceptable. I edge up the recommendation a bit in order to provide a bit of additional output and low-frequency extension. You can go even larger, but in the car audio environment its not necessary. For home audio, I prefer to aim for a Q of ~ 0.5, which would require around 4.6 cu ft net volume per RL-p15.

THanks for all the help everyone, now to just find the time.