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.
Using a 15 inch woofer to do mid range
I have noticed that Tyler Acoustics are using a 15 inch to do midrange down on their Pro- Dynamics speakers PD30 & PD 15. Isn't this pushing the 15 inch to far?