I have the new HTC One M8, and the music cuts in and out when I play music. It's sporadic at times, and I've had better luck having playing music while the phone is sitting on the passenger seat. If I have the phone in the storage tray in front of the shifter, the audio is constantly cutting in and out. It also gets all crappy if I have the phone in my hand while streaming the audio.
I don't have the same bluetooth problem in our 2013 accord. My friend's iphone 5s didn't cut in and out in my car either.
Subaru doesn't even have the 2015 wrx listed on their bluetooth site showing compatible phones.
On a related note, why doesn't the car automatically connect to the bluetooth device??
This is my first Subaru btw. I'm not sure how frequent Subaru releases firmware updates to address compatibility issues.