I have never owned the engine you are using but for tuning the pipe you can go to
http://www.macspro.com/tuning.asp
The Jettstream tuned mufflers are a great and simple way to go for a side exhaust engine. Their claim and my experience says you will get 1200 to 1400 rpm increase using the correct prop. Check out the Jett web site for info. It seems like I remember reading the OS 55 AX may be better for torque rather then rpm.