gaRCfield,
You will probably get a few different answers to your question. So here is my motto: before all else fails, follow the manufactures recommendations, but that's just me. I have 3 OS 2 strokes, one is the 55AX (on my GP Venus 40) with the bisson pitts style muffler, and like ChuckW says, it does sound very cool. The only problem I have had is trying to run a 12" prop. It just does not like it! This motor was in my Pulse when it went in almost nose first into the dry lake bed (kid's, don't try that at home!). Shattered the aluminum spinner, couldn't find all the prop pieces, and NO damage to the engine. Just inspected it, cleaned it up real good, and it fired right up.