I too have cut the tips off my bisson muffler on my os 160. I'm not sure if I picked up any rpms seems to be around the same. 9200rpm's, kline pump, 18x6 apc, 5/8" muffler extensions. the one thing I have noticed is that I have gotten rid of the anoying pinging sound the muffler makes in the midrange. I've also read it may keep the engine cooler since theres less of a restriction.