I defer to the experts, but according to the white paper, the center to center discussion is only for cone and dome speakers. The edge of the horn is the critical factor for horns.But the real problem as I see it is the huge difference in sensitivity between the horn array and the mid/woof cone array. Balancing this even using active crossovers, would be hard. On would need a power amp that is adjustable. And doing it with passive crossover would just destroy all the benefits of the horns, in my opinion.
eric j