Cannot answer wheither replacing the batting is practical or not, but... as it pertains to the inside textured black surface that shows....
Charles told me about a wheelwell coating called Fusor 805, by Lord company. After putting it inside my trunk for sound deadener and inside my wheelhouses, I concluded that it is a DEAD RINGER for the inside surface of the firewall pad. When dry, it is about as soft as a sneaker sole, and the color is perfect. With just a little experimenting on how to recreate that pebble texture, and it will be a good solution on restoring these things so they look and feel like new.