O.S. recommends the propellers they do because they make the engine run much quieter. You can easily prop the .75 AX to nearly 12,000 RPM when leaned to just rich of peak RPM. This will give a lot more power, but also be a lot nosier. O.S. is recommending propellers that will allow the engine to run well, handle well, develop reasonable power, and still be quiet enough to pass the noise restrictions that are in place in many Asian and European countries. Some places have noise restrictions that are very, very strict, and the engine manufacturers have to account for this if they expect to have sales in those areas.