The problem with firewall penetrations is the hole has to be big enough for the connector, which became a problem with the DB-9 plugs for my Lightspeed ignitions. Another approach is to make holes just big enough for the wires and attach the connectors after the wire has been run. There are several good approaches, including pass-through kits available from ACS or other suppliers. I went with rubber grommets to protect the wires from the firewall and stainless shields to protect the rubber.
But I've already had to replace the firewall after a nose-over and I can testify that it's a pain in the rectum to work with the penetrations. For that reason, I am springing for CPCs (Cannon Plug Connectors) specifically for the firewall. They are expensive but I think it will be worth it. Larger gauge wire for power will have marine-style bulkhead fittings. All fluid and pneumatic lines will have bulkhead fittings. All the major control cables will have eye-ball pass-throughs and I am working on a solution for the small bowden cables.
My goal is to thoroughly seal the firewall but make it easy to deal with penetrations for future work. It's way overkill, though, and I know it; many aircraft are flying with much simpler penetrations.
