How much r-value is added when Styrofoam is applied over exterior sheathing?

We are putting an addition on a stucco home and are considering Dryvit or similar. My client is wondering about the increased insulation value of using 2" styrofoam vs. 1". The walls will be 2" x 6" insulated with 5.5" fibergl with vapor barrier applied, and the exterior will be 1/2" cement board.