I'm convinced that a bluetooth mic volume setting is there but hidden.
2007-2009 Toyota's with almost the exact stereo have a TSB which describe how to access the secret menu. I tried the steps on my non-navi 2011 WRX, and never got there.
I've been complaining about this for a few months now to my dealer service guys and they suggested calling Subaru directly. I spent half an hour today waiting on hold for the privilege of receiving customer support. They took down my info, and said they would look into it. I would hope that enough complaints come in that Subaru would go back to Clarion and get the codes to access BT mic volume.
I don't know if changing the mic would do any good, but it is simple enough to access in the overhead console.