Well, everyone missed the most important part of an exhaust system, the power pulse reflection. Yes, most racing systems do weigh less, are less restrictive to exiting gasses and are louder, but a less restrictive system does not have to be louder. I won't go into detail on that part, but I'm speaking from actual experience with different sytems, loudness and jetting required. As for anyone who doesn't believe me, I'm referring to the difference between the exhaust sustem on the 1979-80 Kawasaki KZ1000ST shaft drive and 79-80 KZ1000 chain drive bike. Exact same engine with the same cam specs, etc, but the larger, heavier and quieter shaft drive touring exhaust made more power than did the smaller, lighter and louder sporting version chain drive.
As for exhaust pulse deflection, every time the exhaust valve opens and closes, the exiting gasses create a pulse that races through the exhaust system, getting cooler and losing velocity. In the older Kawasaki factory exhausts, the small pipe or tube from the headers extended into the primary, cone shaped muffler shell 8-10" or so. The wave would strike a steel baffel, be reflected backward to the front of the primary shell and like a paddleball, then be reflected rearward again. At certain engine rpms, as the reflected wave passed the end of the tube, it would help pull the exhaust out of the tube, evacuating the gasses in the header pipe more quickly (called scavenging), which it would then pull more air-fuel charge into the cylinder.
With racing systems, the shape of the muffler shell, header length and tube length can be sized to greatly increase exhaust gas extraction at certain rpms. Called tuning, the dimensions can be such that extremely effecient exhaust scavenging can be achieved, greatly increasing power output. The problem is that as effeciency goes up, the engine rpm range where the scavenging works narrows. For a racing engine that is run at very high rpms all of the time, that's not a problem. With ordinary use putting around town or tearing down the highway at high speeds, the system that works so well on the race track creates a system that is extremely peaky, meaning you may have lots of power at high rpms but at low rpms a moped could outrun your super race bike. That's because at certain rpms, the reflected exhaust wave is 180 degrees out of time with the exiting exhaust pulse, actually making it more difficult for the exiting pulse to get out. This increased restriction actually cuts power.
There's a lot more to it than that, but that pretty well explains how exhausts work. Looking at the dyno graphs of 1970-80s bikes show the area where power production drops or even reverses as rpms go up due to out of time wave reflection. Modern exhausts often have servo motors that operate devices inside of the exhaust system to increase or decrease restriction, reducing peakiness and eliminating the out or time waves, thus eliminating the dead spot in power production.
Most people know just enough about certain subjects to show how little they know about the subject.