Memphis Sub

This is installed in a 97 Grand Am Se without folding rear seats.

Ok. I've got a single Memphis Audio 15-SHP15.
Specs:
Znom: 2x2ohm
FS(Hz): 29.7
QMS: 4.8
Qes: 0.352
Qts: 0.33
Sensitivity: 93dB
sd(m2): 0.079
vas (c.f.): 2.72
Power: 500Wrms/800Wpk
Diam (out): 15"
Diam (in): 14"
Depth: 8.25"
Disp: 0.29

Right now it is in a 2.6c.f. ported box tuned to 38Hz. Dimensions are approx 30w x 17h x 10d. It sounds ok, but I think it could sound a LOT better. When its placed in the car facing the rear seat, or facing up it sounds like crap at higher volumes. You can hear the driver "puck". When you face it back towards the rear of the car it sounds real nice. No distortion at all.... so its not that its being overdriven, or underdriven.
Should it make that big of a difference which way it faces? Is it because of the seat being too close or something of that nature?
My ultimate goal is to fire the driver driectly into the cabin. I want to mount it in the center of the back seat and also port it into the cabin. I want to build something to do MAX SPL.
What size box (c.f) should I build to achieve this. Memphis's site recommends a 2.5cf box. Sounds small for a 15. Is this the optimal box size for this sub? Should I allow more depth for the sub? It currently only has about 10".... and the sub is 8.25" deep. Does depth matter?
Sorry for the long winded question, but I don't want to put all that work into building and have it sound bad. Any input will be appreciated.

www.powered4sound.com (http://www.powered4sound.com) www.decware.com (http://www.decware.com) I don't like memphis subs.

when you move the sub around, you are changing it's location - line in or out of a pressure node