My 115 efi Mercury (my first outboard) on a 6 metre Valiant won't pull more than 4750 rpm, and I am dissapointed with top speed, about 35 mph with light load and calm sea.
Am satisfied prop etc correct, and have been told it (engine) adjusts itself according to load!! It's very quiet, reliable, and doesn't seem to use much fuel, albeit I've no idea how many gph.
Although not much help, I would question it's underlying power compared to a two stroke and given a choice, I would have gone for a bigger engine. It was the biggest 4 st. merc. did at the time, and was very cheap. (Whopping great discount from Valiant who are owned by the same corporation that owns merc/mariner/Johnson/Sea ray/Bayliner/Quicksilver etc etc etc.