Great point about the tri-toon. I wouldn't own a party barge with only two pontoon's. Makes a HUGE difference, especially when you have it loaded down.
As far as I/O, there are several around here with 496s. For my uses the outboard works better. Way less draft, easier maintenance, less fuel, cheaper, and not much difference in performance. On the Southbay's you have to remove the fuel tank to change the water pump impeller. Not something I want to deal with. Plus, after a few minutes the "cool factor" of having straight exhaust on a 'toon gets old and you end up using the silent choice. I'm an I/O guy on performance boats but for the 'toon I'm sold on 4 stroke outboards.