The TT muffler is quite a bit larger than the stock muffler, and the outlet is larger. I found it ran a few hundred rpm better with the TT muffler than with the stock muffler, which I think is very restrictive. Didn't you do an adapter to run your Enya 30SS with a 40 size muffler? The Enya 30 has the same bolt spacing, so why not give your OS the same treatment?
I'm away from home, so don't have my figures in front of me. I remember liking the engine a lot, but from figures I've seen it is not nearly as powerful as the FX.
Jim