Hi Jeff, the rule of thumb I noticed was that each movement in pitch (up or down) will affect your theoretical speed about 6% at a given RPM range.
Its the propeller RPM that will determine your speed (assuming your motor has sufficient torque to spin the selected prop at low and max rpm,)... so when you mention engine 6150RPM... you need to take into consideration the RPM reduction in the lower leg gearing ... usually somewhere between 1:1.75 --- 1:2.08... then you will have the prop RPM.
Here are two sites you can model the effect of spinning different pitch propellers.
Simple MAX theoretical speed calculator (enter prop rpm and pitch)... You can use this to work out what the max speed is, and then what you are observing and hence your propeller slip factor....if its more than 15% you need to tweak your prop set up.
This is a more complex calculator where you have all the relevant variables - it will calculate the answer for whichever variable you leave blank.
I used both of these sites extensively when I was modelling my repower.
Regarding your mates boat - are the propeller diameters the same? Are the boay weights and weight distribution the same. There are many many variables. When I spoke to sites that had repowered boats like mine and prop pitch selection, they gave me an example of two identical boats (in that case Carribean ReefRunners) where one was successful with a 16p and the other 17p. The only difference was how the owners had set up the boats.
Hope this helps.