i x2 talking to spkrman, ask him about 3 10's vs. 2 12's, he's prob gonna wanna know how much room you have to work with, too, so measure that.
He could probably get one of those alpine 10's to wang harder than the 15" rockford in a prefab
And have you ever been in a vehicle designed for SPL comp's? It's not really the most enjoyable setup for listening to, and louder db doesn't always 'seem' louder, so it really depends on if you're actually going to compete.
But either way, he's your man for type-r's, he loves them.
And also, for SPL, it's less about the size of the speaker, more about the box, tuning, and install. You could get a 10 in a huge box, tuned high, to meter higher than a bigger sub, in a smaller box, tuned low. The bigger speak might sound boomier to you, but that's not what SPL's about.