Interesting, I was in the mobile audio business for almost 20 years - and all things being equal - the same high current amplifier is going to get more volume out of the 18" woofer than the 10" - if the impedance is the same and the sensitivity/efficiency of the speakers are equal - the 18's will be louder - they move more air. Now will they be as accurate? Hell. No. I used to build sealed boxes with 8" and 10" woofers and they didn't make as much noise as a ported 12" - but they hit hard - they were faster and more musical - and they took more power to drive.
So now I'm really confused.
With all things being equal, I think a sealed box is the most accurate but takes more power to equal a ported or band pass box to equal the same DB from what I can remember.