I ended up going the full Shelby/Kicker route, adding the amp, rear speakers and trunk sub to the Shelby door speakers and door subs, I'd already installed. The amp made a big improvement in clarity and the sub really filled in the base nicely. I noticed when using the fader to hear the rear speakers by themselves, that the front subs and the trunk sub are also reduced, along with the front 6x8s to the point of complete silence if you adjust fade all the way to 100% rear.
If I were doing it again I would skip messing with rear speakers completely, they don't seem to add much and are overpowered by the front speakers and all three subs.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade