It all has to deal with the transfer function of the car. Most cars are set up so that the subs work best facing the rear. My 5th gen is like that. If I fire the subs forward, the roll off of the subs is very high and I don't get much deep bass. The bass is tight and accurate, but there isn't much sub bass at all. Firing the subs backward (toward the trunk) gives a better low end frequency response to the subs and also gives more volume. The bass isn't as tight per say, but still sounds good.
To find out for sure about your car, the only way to do it is to try it.
Mish