The firewall is total overkill. Instead of figuring out a way to lighten the tail the designer decided to sort of take the easy out and add a small brick to the nose. I have never seen such dense plywood. On the other hand light ply firewalls can be nasty in their own way. I have had a couple delaminate on me over the years. If you do use a light ply make sure to do something at the corner joints to incorporate all the plys into the joint. My favorite method is to cut thru the corner joints with a fine saw and then cut triangles from medium thin carbon mat (.0125) and glue them into the saw kerfs. Very light and very strong.

Bob