Hi Scootmeister,
All types of IC motors benifit from tuned exhaust systems. Exhaust upgrades are called "cheap HP" because the gain is high in ratio to money spent.
The reason I hold so many speed records is because of tuned exhaust and understanding drive ratios. My 1999 Whizzer will exceed 68 MPH with a reverse cone tuned exhaust pipe, and slows below 62 MPH with the factory system. This equates to an almost 10% increase in top end speeds due to the pipe.
The Ducati "Desmo" singles I raced in the late 60's used the same type system and added almost 900 RPMs to the top end and is the reason I use a similar system on my high speed Whizzers.
The 2 strokes I raced [Bult, Kaw, Yam, Carabela, AJS, etc] had "expansion" chambers, and of course added large amounts of power [cheap power].
4-stroke pipes can add a lot of power, however if done wrong it can reduce power. The most common mistake is size and length, and bigger isn't always better. If you look at the HS 142 motor, it is clear the exhaust has been "downsized" in the process to reduce the sound. Even the pipes we include in our EZM kits, while better than the "box" muffler, still reduces the output in a major way.
Part of the tuning requires an understanding of "pulses" which are based on pipe I.D. and length. Once the correct I.D. is determined, the desired use will effect the length. As an example, when racing Ducatis I used a long pipe with a long reversed cone for long tracks [road racing], and a short pipe with a short reversed cone for short dirt track racing [short tracks, 1/2 miles, scrambles, etc]. Short pipes move the torque curve lower, add more mid range, but lower top end speeds. Long pipes will slow the RPM rate, move the torque curve upward, but adds top end speeds.
Most often "tuned " exhaust systems are an average to make the motor more user friendly.
The pipe construction is also critical, as the exhaust must exit smoothly, and the I.D. surface shouldn't disturb the "pulses". An example is using "LP" gas line, or water pipe, as the interior isn't smooth. Another issue is bending too sharply.
To help you get started with the HS 142 motors, start with 7/8" I.D. pipe. Must be smooth interior pipe or automotive flex pipe. Some "back pressure" is helpful for mid range, and can be a straight tru muffler or a specially designed tip. Length will depend on desired useage.
Have fun,