What is your opinion of box extenders like the one pictured on this link? When a wall or ceiling is sheeted over, do these devices satisfy the requirements of article 314.20? I've always thought no. If the box is mounted in noncombustible material, and is less than 1/4" back-set, they wouldn't be necessary at all, and if the box is mounted in combustible material it must be mounted flush. If the box is set back more than 1/4" in noncombustible material, or less than flush with combustibles, I've always assumed that the box needed to be moved out. I realize that in most cases the AHJ would probably allow this, but is it technically a valid fix? Article 314.20 has no exceptions.
I have used this type of extender and they work pretty well. I think that they are allowed and are one of the few ways out of the issue of having a box set too far back. When I first started the only extenders were sheet metal, they also worked well the biggest problem I had with those is that the metal ended up very close to the energized screws so I always taped the device. You are correct the best solution is to have properly installed boxes but I for one don’t live in a perfect world.
If you notice from the page they are CSA certified but not UL listed. Some AHJ’s would reject them based on this. I have had inspectors accept them in non-rated walls but not firewalls. What that decision was based on I am not sure. A call or e-mail to Arlington’s tech support could answer all your questions. If you do, and get an answer, be sure to post back here so we can all learn the outcome.