To get the maximum acoustic loudness per watt look at Bill Fitzmarice cabs
There is no such thing as a "Horn effect" other than horns couple the driver better to air than a regular baffle, you get a lot more volume per watt. Read about it at Bill's site.
Too bad manufacturers don't actually publish measurements and charts. There'd be no question on which had a better curve for what you want to achieve. And construction, design, power, and price, would actually make more sense in comparison. It's done for FOH cabinets, but bass players don't demand it for bass cabs.
“Please don’t make the mistake of thinking that the arts and sciences are at odds with one another. That is a recent, stupid and damaging idea..." Tim Minchin