I was pretty sceptical when Iread the manual myself, their reccomandations went from 15x8 to 17x6 for sport flying and 13x11-13 to 14x10-11 for aerobatic flying. Iwent right down the middle of the sport flying recommendation and did a 16x6. Can't remember what the tach reading were, but they were fine and Ilike the way it flies, engine response is great, so I haven't bothered to change it.

Anyway, it's an easy enough fix to try. Put on a larger prop, see what happens.