Is there any upgrades for the inner door membrane? Mainly want something better for sound quality reasons. Was going to try fitting the metal inners off a mk4 and sound deaden them but can’t find anything online that says they will fit. I’m aware the window regulator is attached to them on a mk4 but I’d just remove them and keep the original if the actual inners fit 

I would make sure the membrane is sealed properly against water ingress into the car, and if you want sound deadening apply it to the inside of the door card,

The door structures change a lot between the different series, I wont say it is impossible but the amount of cutting and fabricating, it is not worth if for the gain.

I suspect the door seals are more of an issue for noise now, rather than the membrane.