406.4(A) says that the receptacle "shall be installed such that the mounting yoke or strap of the receptacle is held rigidly at the finished surface."
If the receptacle is held by the 6-32s so it's tight against the sheetrock and doesn't move around, it satisfies 406.4(A), even if the receptacle isn't tight against the box. And if it's the self-grounding type, it seems like it'd be effectively grounded according to 250.146(B).
However, 314.20 says that you still need a spacer if the box is set back farther than 1/4" from the finished wall surface.
All that said when I did maintenance work 95% of my receptacle repairs were because the box was set back from the sheetrock and there was no spacer for the receptacle. Eventually the sheetrock would crumble because of the force of people pushing plugs into the receptacles, and then the receptacle wouldn't be mounted to anything.
It's a ratty install. Use some sort of spacer.
-John
