Once more material than allowed is removed from the head. The valve clearances to the pistons are reduced further than Mercedes wants. Another thought was it also reduces the chamber volume as the depth of the valve pocket is reduced. Using the gasket also allows you not having to shim up your pre chambers.

The swept compression from the slight additional thicker head gasket is probably not signifigant enough to worry about. Or if there is concern about it remove almost the 2 mm from the head would return you close to the normal situation.

You could never mill the head again though. You can spend as little money as you want or as much as you want. At least in this way we have free choice. Cost of head material removal is probably flat rate. All the information about using the thicker head gasket should be somewhere. Perhaps even from the gasket manufacturer.

I have no ideal if Mercedes allowance for head thickness reduction is written in stone but feel it is wise to respect it. You are over it obviously.