Managed to patch up about 80% of the firewall, drove 250km yesterday and it made quite a noticeable difference. On the freeway I can now actually hear what they say on the radio

The stuff costs about €18 for 5 pieces of 25x25cm and it's totally worth it!

Good to have reports of the use of soundproofing. Are you planning to go further - inside doors, quarter panels, etc?

I got so called 'egg mats'(dunno if that is the correct term) behind the door panels:



They are there for more than a year now but no sign of mold or moist yet...
They don't have that much effect, but when you shut the doors they sound less like a tin can and they are utterly cheap.
Next things to tackle are the inside of the triangles and and wings because the motor and box make the most noise.