I'll clarify.. I've put this in construction as well due to the 2 inch difference between wall depth. I thought both topics would apply.. sorry if I broke a rule
Nope, no rules broken but the thing is, it's the same people viewing all threads. When a single topic is opened in more than one thread it starts getting real confusing when comments are made in both threads without knowledge of the second thread. No big deal.
I'm not sure how well joint compound will want to stick to the foam. So...I would use some spray adhesive to spray the juncture. Then after that adhesive tacks-up apply some mesh seam tape. That way the joint compound won't try to separate from the foam. 3M makes a spray adhesive for use with drywall components.