This is from Burns Stainless (who make pro racing exhausts) They don't even mention or sell Mild but do have 2 different Stainless Steel tubing.

304 stainless steel tubing combines excellent physical properties with a remarkable resistance to corrosive agents found in automotive exhaust emissions.
304 tube is the most cost-effective grade of nonmagnetic stainless steel for general applications, but some professional racing teams use the higher heat-resistant properties of aircraft grade 321 stainless steel.


321 stainless steel tubing is one of the most desirable materials for exhaust systems because of its excellent resistance to fatigue and cracking at elevated temperatures.
It is usually the material of choice for high temperature turbo applications, unless extreme requirements indicate the use of Inconel.


625 Inconel, containing approximately 60% Nickel and 22% Chromium, was developed to cope with the extreme heat and strength demands of jet engines.
Molybdenum and Niobium elements also contribute to 625’s strength at temperatures greater than 1800º F, where 321 SS is unsuitable.
Inconel 625 excels in applications such as Rotary engines and extreme duty turbocharger exhausts.