I have a 2009 with the 500 and a 2014 with the pro system. The 500 does have door subs that sound amazingly good - do not go for the stock radio or you will be sorry - from all I understand it sounds poor. I'm quite sure the amp for the subs is right in the radio itself. I think the rear speakers are the same for the basic radio as well as the shaker 500 or pro - they are fair at best.
I already bought a 2014 GT Base and there was no option for a better stereo(not that I wanted to pay any more $ for a stock stereo anyway). The Shaker 500 comes with the Premium as standard and the Pro was optional on the Premium.
However, I want to upgrade it a little and was looking into a cheap way to do it for now with stock parts. What I really want to know is what I need besides the door panels to upgrade it to the Shaker 500.