It can't use the same scheme as the 787. The 787 actually removed a layer of insulation that was no longer needed, so they say, due to the non-metal nature of the fuselage.
But there may be places to remove insulation and recontour, which is what the A350 design supposedly does. Even 2" total would help a lot with Y comfort, but obviously 5-6" would be really desirable. Just doubt there is 2-3" on each side to play with.
The unfortunate thing about insulation is that finding a non-flamable, thin, highly insulating material that is also light and not overly expensive is a very tall order. The most effective insulation is a honeycomb of space filled with no air. The second generally is a honeycomb of spaces filled with air. Loose fill and batt insulations are basically this type of insulation. But you need to keep them loose and provide them with space for them to work. Reflective coatings can help with radiation, but conductive energy transfer needs to be block by trying to eliminate the medium by which energy can conduct, that being matter.
Of all the things to worry about... the Wookie has no pants.