well I know that you can put 8" slim subs to replace those midbass drivers in the doors. It'll take some effort to make slim subs fit, but they'll fit.
I have a friend who has a 2008 california special mustang gt and he wants to upgrade his sound system which has the shaker 500 in it. I told him to upgrade the speakers first before doing the head unit as he used to have a 2005 mustang gt with the shaker 500 and he put in a kenwood head unit. He likes to crank up his music loud and first off, I knew that he had blown his factory speakers being that they put out about 5 rms watts and are paper material. The kenwood put out roughly 22 rms watts per channel.
But for his 2008 CS mustang gt, he wants to keep the factory look and was wondering if you could add an aftermarket amp