I've got the Shelby Kickers running off the OEM amp on my '13 premium right now. Somewhat less volume but much tighter and cleaner sound. Took the opportunity to install plenty of GTMAT sound deadening material behind the door cards and inside the door while I was at it.
I've been trying to decide whether to amp those subs or add one to the trunk and would be really interested in what you come up with. For the 6x8s they have all been replaced with the Shelby Kicker upgrades and I'll probably add the Shelby amp to power them in the future. In my '13, the Shelby amp upgrade does not power the door subs (only the four 6x8s), rather it instead supplies a bass signal intended to be connected to a powered trunk sub. The Door subs are left connected to the OEM amp.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade