Thinkin 'bout the piezos. If you use 6 or 8 per side (yes?) then you might try to spread them into 2 groups either side of the Altec horn center line. The idea being that the horn will be a narrow beam at at the high frequencies, so you don't need extra there. Spread the highs around a little to the edges of the horn. Might give a more uniform dispersion of the top end.

A hare-brained idea, but it wouldn't be hard to try!