download a program that models spl
plug the tsp's into the program.
for example, leap will show you SPL per power for a various box alignments.
the rest depends on the space acoustics which could make a difference of over 20dB easy, so its a really vague question. I always find it silly when people compare speakers using SPL records.
if you want to proprietor compare the woofer, take the subwoofers out of the car, put em in a parking lot and measure them again, then we can get a much more accurate "relative" SPL number.