I have not heard the Pi corner horns or any other corner horn, therefore I cannot comment on their sound. I can only speak from my experience with my 'corner horn' (now with a deflector).

I use them active xo'ed at about 65HZ 4th order. They add a lot of crisp to the mid bass, across the whole room, make cello and bass come to life. For low bass, the sound can be tremendous, it's like a 18 wheel truck next to you, you feel that trembling as well as hear it(I don't understand it when people talk about quick, tight and tuneful low bass, the low bass does not seem to have tunes. Even for the mid bass, how the bass sound sounds depends on the instrument). If this represents in any way what a 'apex' corner horn can do, then I agree with you that it's a great idea.

I would like to add two more advantages of corner horns: by their size, their break some standing waves from the corners; their load the room better.