downward firing works fine, as long as the sub's sag number is sufficient.
calculation is as follows: Percentage of Sag = 24,849 / ( Xmax * Fs²) If Sag% < 5%, the driver may be mounted down (or up) firing. (Xmax in milimeters.)
entirely depends on teh vehicle and the install. usually works very well in SUVs should hit pretty hard and help to hide any mechanical motor noise from high excursion subs.