Be interested to see what comes of this, tried a quick search of US forums, but am at work, and should be working.
I have noticed recently that my standard sub bottoms out on some music I play through the system, and have thought about finding a solution some time
In terms of the size of the sub itself, the travel of the sub is the key, so an 8inch unit will shift the air of a 12" unit, but the 8inch has to travel further, this causes all sorts of issue around magnet design and the non-linearity you get with long throw base units. However a good sub amp and speaker set up with feedback to handle magnetic inertia will make the signal cleaner and so the end sound better.
My view is that the car is such an acoustic nightmare (all that metal and wave disruption) and when the car is moving the underlying flexing of the car adds all sorts of other non-manageable bass from a panel resonance etc.
So I would personally just see if you can get a high spec long- throw bass unit that fits into the existing gap.
Having a reasonably high end system at home and having compared it to really high end (10K+ home audio) you start to get diminishing returns pretty quickly when it comes to bass.
A 30Hz wave needs 11.6 meters to operate freely in, and that is a bit of a challenge, because even if you use a folded horn as a cabinet for the new bass speaker, it will not be air-impedance matched by an 11.6 meter space in the car. (Unless of course you tow a ruddy great speaker trailer, you sit it it, and let a chauffeur do the rest).