the 15/10 is the same as the 16/8 ur running now! just a smaller diameter to allow for ground clearance! General guide to 120 prop sizes I've found is 14/10 or 14/12,15/6 to 15/12 and 16/4 to 16/10 but that depends on your application.Contrary to what EVERYONE will tell you,you want the 4 strokes to turn between 9500 and 10,200 rpms on the ground.They all build max Hp in the 10500 to 11000 rpm ranges.with the 15/8 prop u shouls see anywhere from 9700 to 10,000 rpm on the ground.
good luck Art