There are guys running 10 to 15% on the same plane, engine and tuned muffler as you. They have been using the APC 10X9 and 10X10 with the best results with the latter.
It should be turning that prop just south of 14K. A Viper or cleaner plane is about the only time that engine/muffler combo will use a 10X10 effectively(almost too much prop). On most sport planes the 55AX's sweet spot is 14,300 on the ground with stock or tuned muffler. This engine develops the most power at 14,600 due to port timing and falls off radically after that, so if you can get everything together at terminal speed at that point, RPM, prop, pipe/muffler etc. you know you are there.