I just upgraded my 2015 SLT with Bose by adding Infinity Kappa 6x9 in the front door and 6.5 in the back door. I bought both from Crutchfield after visiting their show room in Virginia (I happened to be near there for business and their testing room for speakers is computer simulated and incredible). The sound is much improved, both range and soundstage. The Infinitys are 2ohm speakers so they work well with the Bose amp and there is no discernible power loss. Most speakers are four ohms but the Bose systems are either 1 or 2 ohm amps, so the 4 ohm speakers display a great deal of power loss.
I have not upgraded the dash or the back pillars and would love to know if anyone has had luck with those.