Interesting timing on here... I just saw this in the prelim Fastrack:

"Spec Miata
1. #6400 (Todd Green) Seeking rules clarification/suggestion for Spec Miata
In 9.3.24, add the following to the end of the paragraph: "Insulation or heat-resistant material may be added to the
interior (cockpit) surfaces of the firewall, floor, and transmission tunnel."
(Please see letter #7625, Minutes for Recommended Rule Change portion of this letter)."

Not sure what that last part means regarding another letter though...