Generally, our ears and brain will not be able to tell a significant difference especially in a small car like the Z. Technically however, it should sound louder. In the Z, hard to tell exactly by how much unless you measure. If you use an RTA, you will see a clear distinction. There is a tendency for most lower frequencies not to be affected whether the sub faces up or towards the rear. Not sure exactly what it would be for the Z. Acoustics make a big difference. A point I'd like to make, you cannot rely entirely on on sub positioning and should more importantly take into account your enclosure, gear, and processing. My suggestion is to experiment positioning before you lock down on your design. Just want to share a couple of tech papers I have relied on in the past.
http://www.sound-better.net/Tech%20P...aper%20101.pdf
http://www.sound-better.net/Tech%20P...aper%20103.pdf