I added a layer of Insultex (basically a thin PE foam with 2 layers of scrim quilted to it) in between the layers. It definintely made a difference, but I think the PE foam would work better as an outside layer. It would trap moisture, though, so high-humidiy trips could suffer condensation as a result. From what I've read, most winter campers don't see condensation issues with Insultex or other vapor barriers due to the extreme low humidity... but I could be wrong.
The other thing I did with my PLUQ (the non-sewn variety) is make it with a differential cut: the outer layer is 1" longer than the inner. This ensures the little insulation you have isn't compressed, and a small air-gap should improve the insulation value. So far, it has worked well for my son down into the mid-40's, though I'd bet with him we could go lower... he's a human space-heater.
"Do or do not, there is no try." -- Yoda