Ok so keNwood are the people that make the disks, adapters, and everything else Subaru can't put their name on. All their navi decks are pretty good, and the sounds are really nice too. (after all u can only hear as good as the deck can make things sound)
So kenwood have decks that can learn up to 8 stearing control features which covers us. I wanted the stock look so I didn't go there. But as for the speakers, I went dynAudio components for the front and left the stock speakers in the rear, added a jl10w6 sub, all powered by 2 jl mini amps. To make it work with the stock deck I added an audiocontrol Matrix. In all on the lower side of 2000. I have great tunes when I go through the USB in the arm rest and Bluetooth. Playing radio sucks though because the stock deck gets shittie signal to pass through the speakers. Mabey the 2015 wrx will have the taken care of. Good luck. If u are near Alhambra "the car shop" is the place to go. They have helped with most of my mods so far.