You have some of your facts skewd.
"Hi-Power Exhaust systems are made from SUS304 stainless steel muffler and tip assemblies." (hksusa.com)
"All of our systems are manufactured individually by hand of the highest quality stainless steel or aluminized steel." (brmexhaust.com)
"Stainless steel resist corrosion and flame erosion (burnout) but is expensive, difficult to weld and more importantly, has a relatively low heat transfer ability. Stainless steel has about 1/4 the heat transfer ability of carbon or aluminized steel. Stainless steel also has a high coefficient of thermal expansion which means that it is more likely to warp and get out of shape when it is heated.
Carbon steel will rust and may be subject to flame erosion over a long period but it is less expensive and is a lot easier to cut and weld. Carbon steel sheets and tubes may be three times thicker than stainless steel, yet still less expensive and have better heat transfer characteristics than stainless. Some of the higher stack temperatures we have measured have been with all stainless steel heat exchangers.
Aluminized steel is just carbon steel with a thin coating of aluminum to resist rust. This is the same material automobile mufflers and tail pipes are made of and has a long record of good service in high temperature applications." (
http://www.tifton.uga.edu/tobacco/metals.htm)
No none of the exhausts that you mentioned is aluminum. Your best bet is to just get the stainless and call it a day. As far as coatings go, I don't think it is worth it to get anything professional done as the cost is not cheap. If anything, you can spray paint it with high temp paint to help protect it from the elements.