So why has lot of people with banshees powder coat their engines without issues?
The Banshee engine cases are built like a tank. Something that has a design that is built like a tank may still be strong enough even if the temper is compromised.
Like I said in an earlier post: Ignorance, luck, possibly using powder that does not require as high of a baking temperature or powder coater rushing the jobs. I was not saying everyone is going to have problems that has powdered coated something aluminum, but the potential of doing harm to the strength of the aluminum is a lot higher than when powder coating something made of other alloys.
If you want the real answer to why some have problems and so do not, you need to study the metallurgy of aluminum and the hundreds of aluminum alloys and all of the different powders and required baking times. The metallurgy course in my Mechanical Engineering studies lasted 2 semesters and is impossible to explain in a forum like this. In general, the temper of many aluminum alloys begins to change when the temperature exceeds about 300 deg. F. It will take longer if the temperature is held at 300 deg than if the temperature is raised to 400 deg. or higher.
Aluminum alloys are one of the most expensive metals to heat treat because of the oven times and multiple procedures required to get the metal have the properties a particular design may require.