It would certainly increase bass response, seeing as the stock system is poor. I would go as far as saying the stereo is the only real low point on the GTS for me. I'm waiting a bit, but will definitely be replacing all the front speakers and head unit. Along with adding a sub or two.
I'm just not sure what head unit to buy as it needs to be double DIN - and I'm hesitant to have someone install it all because last time I had that done in my old car they left marks all through the car
At this stage I'd like to go Pioneer - their high end splits look the goods.