Recommendations are just that, if yours doesn't run well change the settings until it does.
A 17 or 18 inch prop will have more thrust, and will run like it says in the manual, 7,000 to 8,500 in the air...An 18-6 Mejzlik will turn at least 8000 on the ground with the stock muffler..Very good thrust, if the landing gear is long enough..A 17-8 wood prop works well also.....