I'm looking at a single-gang deep outlet box that is at the fill limit- but is still crowded thanks to Mr. GFCI. (NEC really consider a GFCI a single yoke? Counts as two volumes)
Onto the actual question-
How can I get more volume?
Can I get a 4" box in there, either a gangable cut-in switch box, or a 4" handy with a mud-ring? I don't see how drywall ears can help. The only thing I can think of is to open the wall and put in a hanger bar.