"All HKS exhaust systems are engineered for high-performance and durability. Each bolt-on system features high-quality mandrel bent, bake-coated aluminized steel piping and application specific muffler assemblies to maximize horsepower and torque gains." (
http://hksusa.com/categories/?id=1071)
I can see why you think that the piping is aluminized. In otherwords, the companies you mentioned use cheap mild steel and simply coat it to resist rust. So you don't need to worry about coating it. However, you can coat it if it makes you sleep at night. I would be more worried about the rust on the chassis. It's easier to change an exhaust then a body panel
.
When I first read your post, it seemed odd that those companies would use aluminum. First, aluminum is much more expensive them steel. Just another way for those companies to cut costs. Second, it is difficult to weld stainless to aluminum. They also expand at different temperatures which could lead to cracks in the future.
I would also recommend the 85mm HKS piping. Lets say you don't go turbo in the future. No loss. What happens when you do go turbo? Then you will have to get another exhaust.
I'm korean but I am getting a business degree. I'm working on brining a japanese company over to the states.