Insulating behind a medicine cabinet

I will be renovating my bathroom soon. There is currently a medicine cabinet surface-mounted, probably because it is on an exterior wall. I would like to remount the medicine cabinet, which is about 4" deep, flush with the wall. This will resolve the issue that causes the can lights (or any stock fixture I buy) to be partially blocked. I have 2x6 walls, so this does leave a little room for insulation. It seems like 1.5" of polyisocyanurate will come up short in R value: about 9 as opposed to 19 for 5.5" of fiberglass. Is there a better option?