With the standard muffler I got 11,000rpm and with the Turbo-Jett muffler, I got 11,500 RPM with the same prop.
The Turbo-Jett muffler gives it a real nice boost and very crisp throttle response.
500 RPM is a nice boost to you???
The .90 size tuned muffler from Nelson (UltraThrust UT4) will work from RPM as low as ~10,500.
The .90 size JettStream and probably also the Turbo-Jett require a higher base RPM, for the engine to get 'on the pipe'.
For a .91 size engine the prop size range for the Jett is 12x6-13x6, suggesting it will get 'boosted' only if able to spin beyond 12K, with the normal muffler...
...And you are talking about a smaller .75 engine with a bigger, 13.5
A 12.5x6 Bolly Clubman will no doubt show a much higher RPM gain than 500...