Are jbl gt5 subwoofers quite loud to be heard from outside of a car with a ported box?

are jbl gt5 subwoofers loud? if i got a 12" or 15" subwoofer 1st of al how much of a difference is their between the 2? a 12" and a 15" and 2ndly if either of them were in a ported box in the back of a corsa what would i have to power them at to be able to get the most out of the subs without damaging them or the amp? all in all are jbl subs quite loud and is the b any good on the gt5 as this is the sub i am looking to gett for a lot more thump than my original car speakers.... thank you:)