Your car is even newer than mine, I think the system in the car is pretty nice, but if your used to subs, I understand. I would recommend leaving the factory unit but getting an adjustable line out converter that you hook into the rear speakers. This creates a set of "preamp" outputs for an amp then you can hook in your subs. Another good idea would be to get an amp with a remote gain control so you can turn the bass up and down seperatly from the radio bass control. I haven't done this in my car yet, sold my old subs and amp but I have done it in many vehicles before and will be doing it mine soon. Good Luck!
If you can find a faceplate/adapter plate for a head unit that isn't compltely ugly I'd put in an aftermarket one. Even good factory ones suck and will be annoying to control with amps hooked up. Not sure if that's availible for your cars yet cause they look like damn near everything is all integrated into the dash.
I'm loving my Alpine MRP-M850, powers my 2 12's great and those amps go pretty cheap on eBay now. Depending on how much trunk space you want to lose, pick a box size then look at subs that'll work right with it. Just stay away from Pep Boys...
this is such a commonly asked question... what should i put in my car to make it the loudest?? absolutely anything!!! even if you dont buy the most expensive sub in the world...say a blauponkt over a massive subwoofer, or kicker over pioneer... the only thing that makes the difference is the enclosure that they are in, if you build the right box, you can make a $100 subwoofer sound like its worth like 5 or 6.... if you want advice from anyone, and you really mean to take it... you need to tell us your budget.. so we know exactly what to tell you... leave your budget, and i will throw you a list of speakers that will give you bang for your buck