I have the same situation as you, and used plastic washers and plastic screws, to keep the outlets isolated from the box. Worked fine.
Grounding Metal Outlet Boxes and Isolated Ground Receptacles
Just installed six really nice separate dedicated 20 amp lines (with 6 awg) for my new dedicated listening room. Decided to use heavy metal outlet boxes so that I could make my six Furutech Receptacles as strong and sturdy as possible. I also had my electrician run 4-wire 6 agw to the boxes so that we could have one ground wire to the metal receptacle box and then a separate ground wire to the isolated ground screw on the Furutech.
I sent some photos of the setup to a friend of mine (who just happens to be an electrical engineer) and he raised an the issue that since my Furutech Receptacles are metal and they will be screwed into the metal box with metal screws, then we have effectively now tied the two separate grounds together! Help! Is that a bad idea? Is having the box and receptacle setup in this way going to cause issues once my gear is in place; ground loop hums, etc?
I sent some photos of the setup to a friend of mine (who just happens to be an electrical engineer) and he raised an the issue that since my Furutech Receptacles are metal and they will be screwed into the metal box with metal screws, then we have effectively now tied the two separate grounds together! Help! Is that a bad idea? Is having the box and receptacle setup in this way going to cause issues once my gear is in place; ground loop hums, etc?