On the first day of using my Mariah I/O boat, at about 40 MPH, I was talking to my neighbor/passenger about possibly adding the Capt's Call exhaust. He whispered "But then you wouldn't be able to hear this". I agreed and did not go for it.
I can tell what the type of power boat and exhaust system is going past my dock (1/8 mile away) by just listening to the engine sound with eyes closed. If I can hear the engine plainly it is an outboard, with subtle difference in the 4-strokes. If the sound is loud, it is an inboard or I/O with straight exhaust. If I hear nothing and have to open my eyes to look, it is a regular I/O.