There are other things to consider in addition to HP when thinking about speed over ground - the age of the motor and how well it is running, motor mounting height, the prop, condition of the hull bottom, etc. . Often, little effort has been made to dial a boat in to get the most performance it offers. A difference of 50 HP will increase speed, but so will a better prop or a clean hull bottom.
Other than cost and weight, more HP is great up to the maximum HP rating.