I ended up changing all the speakers, including the door subs, adding a trunk sub and a Shelby / Kicker amp. Sounds great now. Mine is a 2013 but I think Shelby / Kicker make kits for the 2011 too.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade