No offence, but this is where you are way off base.
Sensitivity of a loudspeaker has nothing to do with maximum SPL output - that is all down to power handling and total peak to peak excursion. You can have an efficient PA driver with 99db sensitivity @ 1w verses a high power car audio driver which has say a lower 93db sensitivity, but the difference is the car audio driver can have over 4 times the excursion - PA drivers are always limited on Xmax(coil out of magnetic gap excursion) and the drivers I have instance have quad voice coils, and can handle over 10,000w RMS. You do not find this level of specification for a single driver in the professional audio industry.
Also ‘sensitivity’ is measured as an average. PA drivers always come out with a higher average due to the drivers having a higher free air resonant frequency and can produce more higher bass and even lower vocal frequencies, hence more sound, higher average sensitivity. Car audio drivers for pure sub bass application have much lower free air resonance (fs) around 20-30hz, heavier motor structure, kevlar wound cones, multiple coil assemblies etc which lower the higher frequency sound output - but it is irrelevant as you would be filtering these frequencies off anyway when used for the application intended.
As for ‘if you took the car audio system out of the car it would be very underwhelming’ - again, the current world record holder vehicle has over 100,000W RMS. When you open the doors on this you still have way over 160db coming out of the vehicle. Again no PA system has ever hit these levels.
You talk of horn loaded enclosures, but the whole vehicle acts as completely that. The moment you open the doors, the whole vehicle is a horn loaded design. You have a wall of subs, inside an enclosure (the car) and the doors act as the horn exit/vent/port. There are vehicles out there, that can run over 150db, even at very low frequencies such as 20hz - again, PA systems, drivers and the horn designs they use don’t have the ability to drop this low and handle the power.
Again, in comparison, that Void PA enclosure you linked to runs 2 x 18" subwoofers, can only handle 2800w and is made out of 18mm plywood and max’s out at 138db. In the Golf I have two 15" subs, in a horn design, running 20,000RMS, in a 50mm thick birch ply enclosure and this makes 168.8db.
168.8 db is 1024 times louder than 138db, to put it into context.