I have the Mercruiser 120 Hp in my Prosport 6.5 Mt
It will cruise at 25-28 Knots at 3300 RPM dependant on the sea conditions.
Top Speed is around 36 Knots at 4200 RPM
This is with a 23 prop and the leg trimmed out correctly
Incidentally the RIB can make 20 Knots in to a head sea of around 2 Mts without backing off
Fuel used is 3 Gallons per hour as expected, A good power unit in my opinion.
Alan Priddy might get 40+ Knots down one of his big swells around Greenland but I dont wish to try it!!!!!!! ***By Big Swells I do mean the sea type***