Quite fundamentally, most competitive SPL systems will not work well as a streetable daily-driven system - too much on the line with respect to port tuning and power.
And similarly, a good boom-car will never work well as a competitive SPL car. Compromises exist between the two cars:
SPL competitors are looking to keep every last tenth of a dB worth of sound energy INSIDE the car so it can stimulate the mic. SPL cars are usually not too loud, if you hear them from the outside - but they hit about 30dB higher than your typical boom car that you'd hear from 5x farther away.
Boom car drivers want it to hit with whatever CD they have, whatever frequency... from low bass make-me-sick-inside, to midbass kick-me-in-the-ribs. That means they need to build a box tuned low enough to handle most of that energy, while still giving them a bit of an efficiency gain.
Note that ported boxes reduce excursion when you play the sub at the box tuning frequency - you might have observed a sub playing loudly at some frequency that didn't look like it was moving much -
SPL competitors on the other hand exploit this. They tune the box specifically to the one frequency that they wish to burp at - which corresponds to some resonant peak in their car they can exploit, and usually up there - above 50hz... creating a one-note-wonder that also creates a great risk of overexcursion at even low power levels below that tuning frequency. Not satisfying for a daily driver, for a real competitive system.
As a result, an SPL subwoofer is one that basically is "all about motor strength", actually benefitting from a high resonant frequency, and who really cares about excursion? It's needed, but what you are left with after fully trading off excursion capability for motor strength is just fine for SPL, thank you.
On the other hand, a boom car sub has a higher requirement for excursion - possibly a lot of it, if you are really going to be cranking it and getting maximum loudness from a single sub - at frequencies that aren't just that one "tuning frequency of the box". It's also going to be able to handle a lot of power - but you'll be using it under its power ratings, not 4x or 5x in excess of it like an SPL competitor would.
If anything, there's a bigger difference between an SPL competition sub and a "boom car" sub, than there is between an SPL competition sub and a SQ sub, actually.
