Yeah this stuff is amazing. If you want some reading check out
https://www.sounddeadenershowdown.com It's the resource i used on my last car. It makes a huge difference, i only did the two front doors and it made the car much quiet. Also as a bonus gave it a nice "thump" when you shut the doors.
Hugely hugely time consuming to install, i would be scared of the labour rates if i was paying someone to do it. Very rewarding if you have the time, because it's something you notice every time you drive the car. Unless you track the car the added weight doesn't make a difference.
Not sure if i'll bother with the Golf, it's very quiet to begin with. Only thing that is tempting is the roof (it's annoying when it rains).