Placing wool behind panels is pretty much products whole idea. Low to mid frequency absorbtion is pretty same that bare wool. Behind panels doesn't absorbt high frequencies as effective, that is usually only good thing when big areas are covered so you won't absorbt high end too much.
Most gyptones measurements are made without and with 2" wool behind panels.
As you can see using 2" wool behind panel will save some space against just huge air gap.