Bob Stearns, a noted marine author, once posted extensive and multiple studies of the same boats tested with 3 motors (all rigged and optimized individually), but all his tests were of 3-cylinder and bigger motors - but IMHO the examples hold true.
In every case, using the example of a boat powered by a 90, 115 or 130, the middle to higher of the range OB motor was always the BEST for all around performance. It had better gas mileage and was only 2-3 MPH slower at most, then that max rated OB motor. Can be cheaper to purchase too!
My last boat before my current, was a 16' tin of max 60hp, but it was purchased with a 40hp. Long story short, but I swapped out my 40 for a friends 60 as his son almost killed himself :shock: on their boat, which was only rated for a 40hp. The max MPH was only 5 MPH faster, where due to running in saltwater rivers I could only run WOT on flat *** calm conditions anyway, never mind that your optimum fuel efficiency is around 4,200 RPMs for a 2-stroke and up to 4,800 RPMs for a 4-stroke. I did to help my friend and it did carry a load (like carrying 6 people aboard) much better, but I only did that a few times anyway per season.
In Stearn's testing, the mid-range OBs always had the best fuel economy, even if a mile or 2 slower cruise than the max HP motor.