So I just posted this on another thread then found this one. It appears you've done the exact thing I want to do as well. Here's my post and could you update as to how everything is working for you now that's its been a few months installed?
"I know I'm bringing to life an older thread here, but did you go with the 8500 and 250? How is it working out for you so far? I'm looking to upgrade as well and I'm wanting to retain every factory function I've got (BOSE, 2nd row entertainment, back-up camera and sensors, bluetooth, sirius, onstar, door chimes, etc.). I use an iPhone 5 now as well. Did you go with the Pac-Audio RadioPro 5 (RP5GM31) interface kit or the Axxess (ADGM100) interface or even something different?"