For reference, I only added stainless to the lower center firewall. Effectively a 24x24" piece of stainless over fiberfrax that covered the area over the tunnel and the likely exit path for any under-cowl fire. Then, I used another 24"x24" piece on the belly with fiberfrax underneath.
I rationalized that the rest of the firewall was unlikely to see the same temperatures as the lower center firewall. I pray that I never have an opportunity to test my assumption.