I have the bison pitts muffler on my os 46ax and did not notice more than a 3-400rpm drop. The power is great and I would not expect the 55 to loose too much either. the sound is so much better than that of the stock muffler and the looks are also an improvement. My 55 is turning 13600 rpm with the stock muffler and I am swinging a 12x8 on it-does not bog down and on a 6 oz tank in my dazzler I get about 14 minutes of actual flight time. I usually run 1/2 to 3/4 throttle. love the ax.
dan