I bought the S5 and M8 to test this functionality, as I have exact same deck (712 HD) as Bearz installed in my Challenger. I can confirm that as of right now, the S5 doesn't support the DriveLink app, assuming that is in fact the only app that performs the mirrorlink functions on the Galaxy phones.
I'd been using and S4 for the last year, and I only cut over to the M8 this past Saturday. On my S4 the DriveLink app would open, but would crash every time you connected it to the deck. The S5 can't even open the app. My assumption is they'll add something later, but as of right now I'd say it's a no go, at least for me. I searched far and wide with the intention of just purchasing the S5 with mirrorlink rumored to be returning, but I couldn't find anything concrete either (like you guys).
Had a little money laying around so I decided screw it, I'll get em' both and just won't activate them, testing them at home on WiFi. Saturday I confirmed that on the 712 at least, the M8 works with no problem. The HTC "Car" app that comes with the phone synced up automatically with my deck, and it's working well so far.
I'm making a video for my brother to help him decide which deck to get, but I'll post a link here too when I get it done.
Dez, with AppRadio couldn't you get the S5, root it and just use AR Unchained? I saw a bunch of videos on this on YouTube and was considering switching to a Pioneer deck if neither the M8 or S5 worked with the Sony deck.