[Blas]

I think the issue here is how you and I perceive the function of the heat shield as used in this case.
You stated: …’ alu heats up faster and the heat conductivity is higher…” And I totally agree with that statement, but then wouldn’t all that additional heat that is transferred so much more efficiently thru the aluminum plate once again be interacting with the footwalls’ fiberglass and ultimately with your feet? Especially during periods of low air flow behind the heat shield plate. I don’t think the idea here is to create a heatsink here with aluminum, the goal is to reflect or slow down the efficient transfer of heat from the headers to the footwell fiberglass And using a material that does not conduct heat as easily thru itself, like stainless steel, prevents the efficient transfer of heat back to the fiberglass on the footwell. We are not trying to extract heat from the headers or area around them and actually attempting to “cool” it.
And you were also correct in saying the stainless steel is a higher cost, has more weight and harder handling then aluminum, but these facts are not relevant to the question of heat transfer are they.

And regarding your opening statement… --------------------------!