Re: aluminum vs stainless for a 15hp
Hitting 25mph with one person plus the load and boat you discussed with a 15hp is real good performance. I wouldn't waste any money on a ss prop on a small outboard. Need the data that others have requested, but dropping pitch 2" - 4" from your current prop is, probably what you are looking at. Your high speed with the heavy load is going to suffer. No way around that without adding more hp.
FWIW: I have a new to me 1648 DMI (a mod v jon boat) w/25hp merc. With two adults (around 450lbs) with minimal gear, no decks, I am getting 26mph gps at wot and rpms right in the middle of the recommended range.