How many times are you going to have 4 divers in the boat, it's a really cramped way to dive.
I had a 12 ft achilles with a 25 hp Johnson 2 stroke, the only time I took it past half throttle was when I wanted a speed rush. I wouldn't do it too long for fear of being bounced out of the boat.
I cursed that 25 every time I loaded, unloaded, carried it up stairs and felt my back twinge when I got into some awkward position. I offered several people with 15's an even trade, nobody took me up. Pick up a few motors and move them around. Portability is the main advantage of an inflatable. Motors are an awkward load to handle, think of what a pulled back would do to your day or week.