I might disagree with El Pirata about a performance loss being a given. I don't think adding a muffler will always result in a loss of power. I was into drag racing for many years (still have a '68 big-block Camaro) and read many articles on the subject of open headers versus mufflers. Most times, if a performance style muffler was used, there would be no horsepower loss on the chassis dyno. Quite often, there was actually a performance GAIN. These tests showed that most (if not all) engines like a little backpressure.
As we all know, the whole purpose of a tuned pipe is to increase backpressure to a specific, optimum level. I'm kinda surprised one of these tuned pipe makers doesn't employ sound-reduction technology in their design, perhaps like Flowmaster performance mufflers, which use metal baffle chambers to cancel out noise, but don't affect performance. I'll bet if enough people started asking them to produce a noise-reducing tuned pipe, they might do it.
I'm hoping one of the RC mags will do an article and use some sort of dyno testing to compare power levels with and without muffler.