Well, you either have a poorly designed box or a poorly designed sub. 15's can still have "punch"....given they are setup properly (and aren't just some cheap, crappily designed sub)
What sub are we talkin' about here? Box? Amp and settings? More info is needed here.
And you are sure you are actually missing "punch" from the sub, and that it's not midbass punch that you are missing (that many people mistakenly attribute to the sub)??
EDIT: Sorry, just realized that this sounded more like a hypothetical than a real world case. Hypothetically, it shouldn't need to be done because, given a proper install, 15's will be just as "punchy" or "quick" (or however you want to say it) as smaller subs. But, many times when people say there is lack of "punchyness", it's actually a lack of midbass that is the problem and nothing to do with the sub. So, long story short....it could help, but it would only help cover up a poor install/setup, which is the real problem.