Thanks to all responses. Decided not to pursue with the soundproofing after much consideration.

Just found out from the manufacturer's website that the vehicle already has some basic soundproofing installed to it as below, although after test-drive it's not exactly very quiet, just normal/acceptable level of quietness and not impressive quietness.

"Further improvements on the NVH level are noticeable through added insulation for all 4 doors, A-pillar, body side sealing and thickened door glass which helps to reduce road noise. All these refinements contributes to a quieter cabin altogether."

I've done it on two of my rides.

Lancer : I did autofoam (ultimate package) and did soundprooofing for the car boot. Modified exhaust, family bising.
Ativa : Only Firewall between engine and cabin not soundproofed. Doors, roof, boot, fenders and bonnet soundproofed.

NVH is a lot to do with chassis engineering, factors like torsional rigidity, more ultra high strength steel in C,D segment over B segment etc. B segment use lower quality/price parts like torsion beam over multilink, different welding/construction, steel pressing, joint design, suspension geometry, etc etc. More bushes better ride will absorb more imperfections hence less NVH. Things like sound insulation engine cover in X70, C segment vs lack of it in S70, B segment for example. Not something you can DIY. Same with acoustic glass but nowadays even B segments like Vios and Omoda 5 get them too but still can't diy it

If just throwing soundproofing can make a car's NVH as luxurious as Rolls Royce then there's no need for RR's existence, just buy a normal S-class and put more layers of soundproofing.

Yes, I get your point.

Do it only if you’re gonna drive that car for the rest of your life or you’re going sleep in it

The best sound proofing is a good car stereo system and your favourite playlist. Works all the time.

Well said. Can’t argue with that logic!