If you are already happy with your current torso/arms insulation, all you need is extra for your head. A down balaclava would probably satisfy that.
Another option is to replace your UL inner with a down parka. That is what I did personally. I had a down jacket, and I wanted more head insulation, so now I use a parka instead. The hood is always attached, so you don't have to worry about it slipping off your head. That is one problem I have found with UL down balaclavas - the fabric is so slippery, you need underarm straps to keep it securely on your head. You don't have to worry about that with a parka.