So basically it's all or nothing? As in, replace the whole system or none of it?
Don't despair! There are changes to the stock system that will improve the sound.
I simply replaced 5x7's in the doors with Kenwood separates and enjoyed an immediate improvement in clarity etc.
The input allows you to run any source you can think of.
I'm not sure whether SYNC will link with a bluetooth GPS, my GPS isn't equipped with it.
Search the forums you will find lot's of info, IMHO there are enough good features with the Shaker500/SYNC setup to make it worthwhile modifying.