I agree phase canceling can be an issue but the potential gains of spreading the horns can be benificial. As mentioned trial and error are sometimes the final determinators.
I did see the tower was curved but doubt the couple inches would significantly affect cockpit volume levels. I am aware of Kicker's attempt to spread the sound horizontal while reduce cockpit volume levels and theoretically energy wasted to open air above. Having that horizonal spread should allow the mounting at a greater angle or further apart to allow great coverage.
I was not as concerned with "Stereo" sound as simply getting sound directed to the flats where most calm riding, thus rider listening occurs.
A wide dispersion may not be ideal while boating in a populated area where the extra sound may be irritating to neighbors. Listening to tunes is one thing, losing the ability to play them completely or making close enemies can be a greater concern.
A slightly lower volume directly behind the boat is of less concern since there is seldom someone back there plus as mentioned, the extra blasting of swim platform while floating can be a turn off or cause to turn down the system as a whole.
Agreed, the chosen set up is not necessarily wrong, but may not be optimal depending on desired results. Just asking if all options had been considered or experienced.