I use a pitts style muffler from http://www.wackerengines.com/

I believe I paid $28 shipped to my door.

RPM increased from 8000 to 9000 with an APC 18x6.

Be aware - sound increased quite a bit. If you fly where noise may be a problem, this won't gain any friends.

The pipes are threaded - makes for less cutting of cowl.