I am in CA and every city has differend requirements. Some say its ok to put it into the firewall some say no way. One city says its ok if you install double studs on each side. I believe the maximum size electrical penitration per. the UBC is supposed to be 100 square inches which is obviously not enough for any reasonable panel. Most of the time we have the builders install one sheet of drywall on the garage wall and then have them install 2 studs floor to ceiling on top of this to create a place to install the panel. Most people don't mind the 18" x 4" bump out on the garage wall. I would check with who ever is going to be inspecting your project to find out what they require. If you have seperate electrical and building inspectors check with both of them. Electrical inspects typically are concerned with electrical codes and not the building codes.