I know this is an old post.......so many questions about this "premium" system though. I just recently purchased a 2015 Sport with the Sony system. I plan on replacing the doors with 6.5" like you suggest. My question is about the aftermarket amp/sub. Did you install yours by simply snipping the signal coming out from the Sony amp, adding a line level converter, and then running RCAs to the aftermarket amp/sub?
I have gone to more than one shop and they all want to sell me on adding a new DSP module like the iDatalink Maestro AR mentioned in JScaro's post from 2018. i personally don't see the point if the factory unit and Sony amp can perform the DSP function and then i can just add a couple of line converters and aftermarket amps to power everything.