Using a 15 inch woofer to do mid range

Are you sure of the crossover points they are using? The horn can cover quite a bit of the treble and midrange.

15 inch won't go more than about 500 Hz before beaming, however, horns are narrow directivity anyway. Done correctly, they will try to match the beaming from the 15" driver with the narrow directivity of the horn to make a smooth transition in the off axis response. Without a third driver it may end up with some upper frequency roll off but it can be made to work with careful design.

One nice thing about this approach is the smaller capacitor that is used to keep the lows out of the midrange (as opposed to a lower crossover frequency). The smaller cap will be more transparent.