I just bought a 2012 Impreza Sport Premium and have had issues with the bluetooth microphone. I found this thread and adjusted the gain to 1 with dramatic results, but my wife and kids still thought it sounded like it was raining when calling from the highway. I just installed a piece of Dynamat Extreme (6.5"W x 7"L) in the dome light cavity above the microphone. I also added some foam on the sides. My wife said this modification has made a huge improvement on the quality of the calls.
Hope this helps anyone else out there with the same issues. I am still going to bring it up with my Subaru dealer to see if anything is planned regarding the mic.