There may be other ways to do this, but what I would do is to start over. If you have a table saw, rip slats off of a 2x4 to the thickness needed to fatten the studs so that they are 1/2" below the old wall level. This would mean the sheet rock will be level with the wall. If you don't have a TS, then try to find lath and/or other material. These pieces would just need to be tacked to the stud, and it doesn't have to be pretty. Another possible approach would be to use screws to sister 2x4 pieces to the sides of the studs to bring the sisters out to the needed depth. I think the shims would be easiest, but whatever method you use, the end result should be the same. It would be very difficult to get a decent looking finish by adding a filler or plaster to the top of the drywall.