The maximum Sound Pressure Level of any given Sub is a product of it's maximal mechanical excursion and it's sensitivity at the frequency that the box that holds it is tuned to.
The only thing that can stop a woofer from getting to this maximal SPL is overheating (if you have enough wattage available) So the real thermal Power handling should be higher then the power required to get the woofer to it's mechanical limits.
To get the highest SPL in a car, you should have the woofer+box that fits the above best, in a car that with equal resonance frequency.
I Know for a fact that the Germans that took the worldtitle last time, did not only tune their box to obtain maximal SPL, but also spent endless time and efforts to change the resonance frequency of their car to fit their boxdesign.
(Ow, and I've not heard the MTX woofers yet so I can't compare to the magnificient(ly expensive) W7's )