The overflow reservoir does not get filled. It should be about 1/3 to 1/2 depending on system pressure.
Head gaskets on that model leak oil, not coolant.
It's really not a matter of if, but when they start leaking. Had you done your research before buying you would know that.
The dealership is THE WORST place you can have the gaskets done. You can expect them to leak again in 20-30k. They use the wrong gasket and insufficient cleaning / surface prep.
You are likely stuck with it, unless you are able to return it, or pay a proper Subaru specialist to do it right with the turbo model gaskets.
GD