I have two Lancairs. My first one is very near stock and built per the instructions with no major modifications. I am using an OS 91 Surpass II and filled the aluminum spar with lead and she balance nearly perfect.

As far as reinforcing the firewall, I agree with Paul (SDCrashmaster). The plywood firewall is on the inside of the fiberglass. Once you bolt the engine mount to the plywood, the only way to pull it out is to break the fiberglass. An incident that would result in breaking the fiberglass around the firewall will probably be a major crash with damage to other parts of the plane as well. I did not add any glue to the inside of the firewall and don't see a need to do so.