I'm using Ed Schillings little fostex horn. It's really not about the possibilty of "louder" that interests me with higher eff. single drivers, its what I think is a better compliment to the Fi.
Better microdynamics, better coherency and more of that 'liveness" that compliments my listening tastes. Less bass, but what is there is more articulate, propulsive. Picking apart though on audiophile terms interests me less and less, and so lets just say I'm very intrigued with this setup. Single driver horns are imperfect in many ways, so if they don't compliment your priorities, its easy to dismiss them. If they do, they can be addictive.