Here's what I know first hand. The Kicker subs are pretty much plug and play with very little mods needed. Great quality and sound. The factory Shaker amps do not have enough to drive the kickers like they do the stock subs. The stocks have a lower ohm rating than the Kickers. Shelby/Kicker also offered a replacement amp for the door subs that gets mounted behind the glove box (Also plug and play). Its a bit pricey @ $400+. I have not been able to find one for less and for that reason I have held off on the purchase. I also know that all of the quality 8" sub replacements will require a better amp.
I've been in conversation with Kicker and have learned just today that the Shelby/Kicker amp only powers the four 6x8s and not the door subs. Rather, that Shelby amp is a 4+1 with that +1 being an output for a trunk sub. At least in my 2013 premium, the OEM door subs, even if you install the Shelby/kicker amp, remain powered by the factory amp. I'm working with Kicker right now to identify the best way to add some thump to the Shelby/Kicker door subs that I installed. Trying to avoid installing trunk sub.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade