Dont try to skimp on the fire wall. Especially if your going to run a banger in it. 5/16" or 1/4" AC ply and use glass or carbon fiber fill in the fillets on both sides of the firewall. For servo trays use hardwood cross members or lt ply with a double under the screws. The aramid colored fiber woudl look neat. Hardwood if your using pull/pull setups.
Now if your talking a completely built up composite firewall then go for it. You could use 1/4" end grain cut balsa sandwiched inbetween heavy weave of C.F, uni-directional if you have it. By using end cut balsa it is stronger and will absorb the resin making it even stronger. I dont think it would be much lighter though.

my .02 worth.....