A big advantage will be that your response curve will be alot flatter than your current setup, better for more types of music. You might start hearing more of those rumbling lows than before. I think overall it should sound better than your current setup. A disadvantage will be you won't be able to peak at 35hz and get as good of a score if loudness matters. But since you're moving up to 18's, it might even out almost. Who knows. I think the 15's ported will still be louder overall since you have a nice octoport to go with it. But yeah you will be sacrificing more trunk space which you already know about so that's something to consider. Are you not satisfied with how your current subs sound? Why go through all this trouble again?