Thanks again Mikki!
When reading about upgrading the speakers in any car online, you will come across the words "sound damping" and "sound deadening". Some will even claim that if you don't do this, upgrading speakers will (almost) be a waste of time and money.
During the project of replacing the front door speakers, I had a drive in the car with the door panel and the door woofer dismounted, and I was really surprised how much noise it came from the hole where the speaker should be, especially when driving over some wet parts on the road. Initially I've been thinking, this is a A8, not a Golf from the 70's, it shouldn't need any extra sound damping. The experience with the noise coming from the speaker hole made me reconsider.
After some research I went for a brand called Silent Coat and their 2mm option, and I can say immediately that the sheets are very easy to cut with a sharp knife and also easy to apply at 20°C and no need for a heat gun at this temperature.
Here are a few photos before and after.
I forgot to take a photo of the opposite side of the last one, but damping material was applied in the same fashion as on the last photo, just try not to add anything where it might be tight like next to the window motor or around the edges and so on. This was done at both front and rear doors. I did all four doors in one day, and had a couple of long breaks and I didn't start very early either. Expect to spend somewhere between 1 to 2 hours on each door from start to finish.
Before the first test drive I expected it would dampen the outside noise a bit, but was unsure how much it would affect the sound from the woofer in the doors. During the first drive it sounded like I was holding my hands 5cm away from the head at both sides. As I are well used to the car, it was obvious that the noise from the outside has been reduced from the sides/doors, but at the same time I was expecting a bit more damping. So from a pure noise damping side of it, I would say it was not worth it. Audi know obviously a thing or two about noise damping the A8. Playing music on the other hand gave much more attack and punch in the mid-bass, and for this I think the job of applying Silent Coat in the doors are worth it. Not exactly the result I was expecting, but still a nice experience to learn about. Still have some sheets of Silent Coat I've been thinking to cover some holes in under side of the parcel shelf to prevent air to move freely from one side to the other of the subwoofer, as this will cancel out the bass.
Some last words. In my research for this project I came across many answers in different forums telling that "nope, don't do it", when asked about upgrading speakers, adding a sub and/or amplifier in the D3 A8. Not sure where this claims are coming from, but this thread should be a proof of that it is possible and it's actually not difficult at all.
Thanks for reading!