One option is to double up the 705, to make 8" thick panels, but that's not really a soffit shape. And, 705 doesn't perform as well the thicker you make it (relative to other less dense materials). 3-6" thick seems to be the sweet spot.
So yes, if you "frame" soffits with 705, and stuff behind with regular fluffy fiberglass, I imagine that would work well. Would work even better for bass if you put some sort of membrane on the front face of the 705 that faces into the room.