I don't know much about the larger sizes but on the smaller .17 and .18 the muffler is really too restrictive and that also affects the tuning of the carb. The hole distance on the mufflers of the .17 and .18 are the same as for OS and ASP etc. So that is another thing to swap around that really improves the high end.

The fact that there is still a website (dated 2009) doesn't mean that they are still being made...