Got experienced did a sound proof . 2 cars  local sub 50k and non 150k.

The result very minima result l. More on placebo effect

But one thing for sure...stock sound/speaker much better

I have soundproof my both car, but i only have installed 2 layers in the door. It can help you to reduce the road noise & vibration. When u closing the door, u can feel your door is heavy & the closing sound would have reduced.

For rattling sound, u need to identify from where its coming and fix that part. General soundproofing solutions maybe wont help the rattling sound part i guess.

Use good quality tyre which have low noise. This really helps.

I'm aware of that actually. My remark earlier is on old cars which would have shown higher rattling sound in the cabin due to age compared to new cars, so in my book it is not worth it to soundproof the car since the rattling sound is still there. Soundproofing to make the car quieter but rattling sound from other parts of the car will be louder thus defeating the purpose of soundproofing.

The other factor or concern I've raised is when during the soundproofing process, the work may have caused even more rattling sound due to the work of dismantling panels, seats, liners and stuff, so those parts may have become loose and cause additional/new rattling sound previously not present in the vehicle. Worst is during the soundproofing works the installers accidentally broke some parts or clips when dismantling the panels. That's the whole reason of the poll to ascertain on both positive and bad experiences of people who have gone with the soundproofing, especially on new vehicles as highlighted.