The mousse can muffler (MCM) adds about 1000 rpm over the stock muffler on the 15x4w prop.
To build a MCM you'll need to buy a Macs header and get ahold of an aluminum can that measures 2" diameter and 10" long.
For instructions on how to build a MCM, go here:
http://webpages.charter.net/rcfu/
Look under the CONSTRUCTION link for the Mousse Can Muffler building instructions.