Propeller pitch for *****ing

I have a 70 hp that currently has an aluminum 19 pitch on it. I've looked up the range of props I could run and the 19 seems to be the one yamaha reccomends. My friend decreased his pitch and told me it makes the engine run better at *****ing speeds, but loses top end speed. It seems to be a science to figure out rpms and proper diameter/pitch. What exactly will dropping to a lower pitch do?

[boats.net]

Dropping a pitch size will increase rpm's about 400-500 rpm's at wide open and may loose mph but increase hole shot. The operating range for a 70 2 stroke is 4500-5500 rpm's and you want to try and stay in that range, if you put too small of pitch prop on it may activate the rev limiter(about 6200 rpm's).