Ok I got my mustang with a Shaker 500 too and upgraded the head unit to the Pioneer Avic D3 I had in my old car and I love it. If you want to change the head unit go for it since the only thing special about the Shaker 500 head unit is that it has Shaker 500 printed on it and it can support everything thats in the car. Your car comes with 2 subwoofers in your doors which I find have quite enough power. If your want more and you want to go cheap go with a bazooka tube. They are cheap and easy to install and come with the amp built in as well as an enclosure so you don't have to worry about mixing and matching parts. I recommend the D3 but if you dont want that you should definately get a head unit thats a size Double Din like the d3. They're more expensive but they fill up the entire space that the original head unit occupies and they look amazing. Also the sound given out by the d3 is much better than the stock head unit. I say go for it and if you install it your self let me know if you need any help.
i have a 07 v6 mustang and i wanted 2 get a system put in it .. but it already has the shaker 500 so every where i go they give me this big spill about how i should just leave it the way it is i want a indash screen with navigation but i have no idea wat to get as far as the subs or amp go any suggestions? 
