I was just thinking that instead of puttingg steel wool inside the muffler, this might be one instance that an exhaust diverter would be usefull.

http://www3.towerhobbies.com/cgi-bin...&I=LXL378&P=SM

I have noticed on my engines that it will decrease the top rpm, but if you are not getting enough backpressure, it will help.

The problem with steel wool is that it will get gummed up with the oils and not breath anymore.