In a sense, the term "performance muffler" is almost an oxymoron. "Muffler," or "silencer" in the United Kingdom, refers to the suppression of sound, but that is a secondary consideration in a performance setting. The same things that a muffler does to reduce noise are also factors that can inhibit an automobile from being all that it can be in terms of power. Therefore, a performance muffler is basically a compromise between the two extremes.
In a standard muffler, the rush of exhaust gasses and sound waves enters a central chamber that contains a set of perforated tubes. These tubes delay and deflect the sound waves into two competing vibrations that essentially cancel each other out. The result, if all goes well, is a whisper instead of a roar.
The problem is that this single entry point sometimes causes a backup of the exhaust gasses, much like the traffic delay just before a tunnel. That's called "back pressure," and it is what a performance muffler is designed to eliminate. One way of doing that is to provide several entrances for the gasses and sound, a "flow-through" technology that allows freer passage for the former while still dealing with the latter.
There are roughly three classifications of car owners as far as mufflers are concerned. Some want as little racket as possible -- and are, in fact, quite concerned about violating anti-noise ordinances in their communities. Others like the healthy growl of a less-restrained performance muffler, while the hard-core custom car owners don't care how loud it is.
Indeed, that macho growl is a selling point for many factory mufflers. Some advertisers compare it to music. The Web site of performance manufacturer MagnaFlow®, meanwhile, even has a feature with which you can type in the make of car you own and hear on a sound file what it would sound like with a performance muffler underneath it.
Then there is Flowmaster®, which has developed a performance muffler that touts itself as the best of both worlds -- increased power and less back pressure, yet greatly reduced noise. This is accomplished with electronic sensors that keep the muffler "in tune" and more closely regulate the device that manipulates the sound waves inside the muffler. The result is generally a bit louder than a factory muffler, yet still well below the range required by most municipal codes.
As might be expected, performance mufflers are not inexpensive. They are generally part of a complete performance exhaust system, which means the standard pipes and muffler must be cut away to make room. Moreover, such systems, like most custom parts, carry a wide range of prices, from $500 US Dollars (USD) to well over $3,000 USD.