First off you need to look at a 2 ohm speaker set up for the front if you plan on keeping the factory amp and radio. Otherwise you will cut the output power of the factory amp in half. JBl GTO or Infinity Kappa will fit with no modification to the doors.

Second, the factory amplifier also includes an eq, compression and limiter circuits, along with a virtual center channel processor.

So the question you need to ask yourself is... is it the overall sound or overall volume that you would like to improve.

If it is sound, the factory components are quite good but the bottom octaves could be improved by adding a small sub system in the trunk.

If it is volume than a larger amplifier piggybacked off the factory amplifier around 75 watts per channel @ 2 ohms will get you move volume from the doors.