To give some feedback here as well:
I have now insulated 4 casings and done a very amateurish test (CD player and decibel app). Silicone joints probably won't help much anymore
-two rooms 3db quieter
-one room the same
-one room 5db louder
Somewhere I once read that a reduction of 5db feels like 10db. Let's see if it makes a difference in everyday life.