There are two different principles here that seem to be a little confused in this thread. A tuned pipe is an exhaust system that will create a standing wave (acoustic) in the pipe at a partiular RPM that results in a reduced pressure at the engine exhaust port. In a properly designed system this back pressure can be below ambient. This reduced back pressure will result in a good increase in power output at that rpm. Since the back pressure can be below ambient, the power may be more than that available when operating with an open port. The pipe is designed to have a fairly broad peak so that the power increase occurs over a band of rpm, but the band is necessarily rather narrow. It is most effective for continious operation at full throttle (i.e. racing).

A tuned exhaust is an exhaust system designed to create acoustic interferance (not a standing wave) over a band of frequences to reduce the noise in the most irritating frequency range. This usually results in less back pressure than a standard muffler which will allow the engine to develop a little more power. The back pressure is always more than ambient, so though the power may be more than with a standard muffler, it is never quite as much as with an open port.

Though they both contain the word tuned, they are two different principles.