Hey... it's like everything else in the building industry. Sometimes, just exploring makes for the best teacher. The more you do, the more you learn how to prepare for the FUTURE situations that you're going to create. It's funny, but since the first time I posted in this thread I've run across the same thing... almost.
The house I'm in was built in the first quarter of the 1900's. Not sure when, but they used the wall board that came in 8' x 2' sections and then they plastered over the entire surface. That's fine, but at some point around 1960, they remodeled this bathroom with today's sheetrock and simply placed it over the top of the old stuff. When I removed the old mortar based tile, it left me with about an inch and a quarter space to fill. I simply cut some 1 by's into strips and furred out the studs, then prepped the surface with mud to create a level playing field and covered it with Schluter membrane. It worked out fine.
Normally, I would have just ripped it out from floor to ceiling, but that would have left me with a big un-popcorned strip along the edge of the ceiling and I didn't want to have to deal with that crap and I didn't want to have to put in crown to cover it up.
Live and learn.
Actually the thickness difference doesn't really have much to do with it, but joints are just harder to get flat because they don't have the inset that the seems in drywall does, now that I think about it, its the proximity of the but joint to the inside shower wall so adding a stud and putting the joint where ya need it so you can bring the tile out past it is the ticket. And yes sometimes we (meaning me) uverlook the abvious, but in this case don't think a shim is the obvious...yeah I know...I need to work on my spelling.