Overall summary: Definitely worth the effort, clear improvement, far from perfect. And this car will always be a noisy Diesel.
In more detail:
Effort: The above work took me about 2-3h of work (albeit split in mini sessions, waiting for materials to be delivered, etc...), and a total cost of 100 EUR. Very reasonable.
Result: Measuring the car, here are the noise levels (no music, no AC) I measured with an app after doing all of the above, on average:
42dB Cold engine idling
33dB Warm engine idling (surprising reduction of noise!)
55dB climbing a steep hill at 60km/h, cold-ish engine
62dB 130km/h highway, mild climb
Frankly, I have no idea what the above numbers mean, and also who knows whether an app on my phone is a well calibrated device, most likely not.
With hindsight, a before and after measure in a controlled area of road would have been a better way to measure.
So maybe the more important is...
Perception - Positive: Very interesting! I do perceive less noise in the car. But as this has been a somewhat long process, due to waiting for materials, I think I got used to it. It is easier to talk inside the car, the reduction on noise from the back was very noticeable after the foam layer, and now after the final foam I also sense a drop in the engine noise, which is the most important.
The best way to describe how I percieve the change is this: the engine noise is the same, but it feels like the engine is located further away. Like instead of being in front of me, the engine is located five or 10m in front of me.
To describe the effect, the analogy that comes to mind is that the "before state" felt like like having a neighbour doing some noisy renovation. And then the "after state" is like the renovation has moved two or three houses down the road. It is still the same nature of noise, it may still be annoying or distracting, but it is much more muted. Which brings me to...
Perception - Neutral/Annoying: Two things haven't changed, and those make the result not as good as I expected.
1. Vibrations. The level of vibrations that comes from the engine into the cabin has not changed. The kind of vibration one can feel in the heel, in the pedal, or when resting the leg on the side. Those vibrations remain the same before and after and affect comfort. This car would not pass the Lexus Champagne Glasses Balance At 145 mph
2. Diesel sound. The nature of diesel noise is more annoying vs gasoline. The noise is more irregular, coarse, definitely not smooth, thus perceived as noisier and more annoying, and the vibrations are higher because of the compression ratio. Here is a good explanation: Why Diesels Are So Loud? Why Are Diesel Engines So Noisy?. And, of course, the nature of the diesel sound doesn't change with my DIY project. And even if I went 10x on noise insulation, the nature of the noise would remain annoying, less precise.
So my perception of the engine noise and vibrations is definitely still there, just more muffled. Which is good, but definitely not perfect.
For the final round of FAQs:
Would I recommend doing it? Yes, simple nice little DIY project that does reduce the noise.
Will I do more noise dampening? No, this is enough. I will not do more DIY and will not go to a professional. Mainly because now it is quiet enough, and also because it wouldn't change the vibrations and coarse nature of the sound.
So what will you do next reegarding the noise?
Enjoy this car for as long as possible and accept the nature of the noise
Consider a gasoline car, and/or luxury car which is more insulated for the next one
And that's it. Now onto the next thing...