Any high performance engine needs a carefully designed exhaust system. Engineers have three main parameters to consider in passive exhaust design for motorcycles. Appearance and sound, RPM of maximum efficiency, and weight.
Titanium is the lightest and most expensive material.
Stainless steel can be beautiful and is less costly. Mild steel is cheapest but can be coated with various high performance coatings for looks and additional power. Chrome is a popular option. The exhaust system can make or break a performance package. Noise has less to do with power than most people think.