Exhaust manifolds can easily get literally glowing red hot... Much much hotter than 400 degrees, I think regular powder coating would burn right off an exhaust header or manifold. Or do they sell a special high temperature powder coating?
I saw a magazine test of ceramic coated vs. raw steel headers where their only concern was how hot they got according to an infrared thermometer during a Dyno run, they were each measured immediately after the pull was made, then again with a 1 minute cool down. The raw steel hit almost 900 degrees and had cooled down to around 500 after one minute, coated hit just a few degrees below 300 and cooled to less than 200 a minute later. However the internal heat of the pipes I'm sure was wicked hot, the ceramic just insulates the pipes basically.
My brother in law had a big block vette where the headers used to glow cherry red. I run sbc made for torque and don't think I make that I make that much heat. I am trying to find out how hot the powder coat can get without damage. I will post whatever I find out. However, I have a set of old headers that I'm going to try it on anyway. I'll post how that goes too.