The difference in under bonnet temps is bugger all. Correctly installed headers won't melt anything.
Mild steel headers last 10 + years without corroding (apart from cracking, which stainless or ceramic coated headers will do anyway if they are going to crack).
Actually, the heat shields are to keep heat IN the manifold, thermodynamics 101.
Coating, wrapping, shielding is to keep heat in. Number one reason. Retaining heat allows exhaust to flow faster and more efficiently. Cold components (relative to the exhaust gas temp) will cool the gas that it contacts, thermal conductivity, and will slow the gas travelling through the pipe/manifold.
Ceramic coating typically increases HP by ~2%, $400 - $500 on maybe 5-7hp increase? Not economical. Ceramic coating on a 1000hp engine to extract an extra 20hp will make a difference down the quarter.
I have found the difference in underbonnet temps to be significant.
Pretty sure my comment about coatings IRT fluid and thermal dynamics insinuates the "inside" part of the exhaust.
Anyways, from my experience, a small outlay on thermal protection from coating or wrapping has a large positive effect in both performance and reliability.
Over the last 20 odd years I've had every manifold/turbo and dump either coated or wrapped, or both, I've also used heat sleeves when needed.
When I'm building I always try and use best engineering principles and will always be happy to pay a little more for it.
In my opinion not using thermal protection is only doing half a job.
There is plenty of science backed evidence that supports my claims, and plenty of DNF's and early failures from people melting stuff.
In the end if you want best engineering principles you pay for it.
$500-800 isn't alot of money in the big picture, and if you want to do it on the cheap, just paint them with VHT paint and use a good wrap.
As per usual, opinions may vary, just like underbonnet temps and budgets.
