Your question about the fire wall is a good one. The wall that separates the garage from living space is a "fire rated wall" or "area separation wall". This wall must have a resistance to the spread of fire of from 3/4 hour to 1 hour depending on building code in the area. Electrical equipment, plumbing fixutres, vacuum systems, intercom systems, etc etc. that are mounted "in" the sheetrock that makes up this wall, compromizes the fire rating of the wall itself. Testing of these walls is done by many labratories around the country such as U/L, ETL and many private testing facilities including Gypsum Association to determine how many "holes" can you put in the sheetrock and still maintain the resistance rating. Holes can be in the form of plastic boxes, metal switch boxes, etc. The plastic boxes should have a fire rating embossed in the back of the box signifying this box has been tested for a 2 hour wall for example. If the box is not listed for a fire rated wall is should not be installed. Also the separation of these boxes is considered in the "listing" of these boxes - only one box in a bay or no back to back boxes can be installed and only so many square inches of "box space" per so many sq. ft. of wall space. Metal boxes on the other hand are tested "in kind" and the restriction for placement of these boxes are generally restricted because each metal box does not have to be tested individually as does the plastic version. You may find the restriction for the metal boxes is more severe than the plastic only because the plastic types have been indivually tested and each has its own listing for fire resistance. Also the metal boxes can be used in 4 hour walls and plastic cannot.
So much for the holes in the wall and the plastic switch boxes penetrating the walls. There are "no" fire rated circuit breaker panels manufactured. I have researched this particular item and have found that there is only one cabinet box that is manufactured with a fire rating and this is for a fire estinguisher.
If you mount a flush mount panel board "in" the wall, removing sheetrock to do so, you are compromixing the integrity of the fire rated wall assembly. You may not get a violation from the electrical inspector or the building inspector because from what I have seen in the industry no one writes violations on suct things. Some architects do not make an issue of this because the flush mount panel looks better than surface mounted panels.
You should mount the panel on the surface of the sheet rock or build what is called a 5 sided box with sheet rock - the same type used for the wall - in the cavity where the flush mount panel will be placed.
This subject is definetly one that hase been ignored in the electrical industry and your question has opened a door - possibly.
This same issue comes up with "fire rated separation" walls between tennants in commercial buildings - how many flush mount panels have you seen in these walls?
