Thank you for your help. Some other details I have located include:
Honda BF15 Yamaha 9.9HT
Weight (H) 52 kg (Y) 51kg
Transom height (H) 703mm (Y) 635mm
Gear ratio (H) 2.08 (Y) 2.92
Prop size (H) 11 3/4" (Y) 10.5”
Difference between the Honda 15 and the 20 is just the carburettor.
A 9.9hp flat out and using the fuel that goes along with WOT (wide open throttle) is making 9.9hp.
A 15hp will make the same hp, and a lot more torque at much less throttle and would use much the same or even less fuel, but on WOT will return a knot or two more speed. One gets power tilt and an extra 4 amps charge, at a price.
I would be interested in the reliability (specifically the later models of both) and any corrosion statistics from anyone. Thank you again.