I had an unlocked S4, which took an unfortunate dive and ended up non-functioning. I used my phone insurance to acquire a new (unfortuantely) updated phone running the full MK2 suite. First order of business was a root & install of SafeStrap, followed by installation of Hyperdrive to Rom Slot 1. I use my phone with my 2013 Subaru XV Crosstrek's bluetooth handsfree for both calling and media playback, and I use this feature EVERY SINGLE DAY. Unfortunately, not any more. I cannot get more than 15 seconds of connected time over bluetooth with the phone. I've tried multiple wipes. I'm fairly certain the issue doesn't exist on stock rom; so I may end up just customizing a stock rom in slot 1 by removing absolutely everything and building it from the ground up, since I don't have to use the (apparently buggy) mk2 modules.
I'm seriously considering selling this phone and getting something more friendly to modding, this is a repeat of the Droid Bionic at this point.