There are a few things to keep in mind
4x6's in general are mostly the same. Pioneer makes some great 4x6 replacements that are going to produce somewhat a better low, but it terms of price v. benefit, save you're money for something else.
Your speakers are also going to be only as powerful as the power your running to them. So don't think your Kicker speakers are going to sound better at 18 watts RMS then your factory option. Same power means relatively same quality. It's once you put those speakers on an amp (Craigslist for $50 EASILY) that the speakers in the dash and the overhead bar will really
shine over the factory stuff. I don't like or recommend Kicker to begin with, but others have had good experiences with them. I stick with pioneers and Kappa.
If you want to upgrade your speakers, first look at throwing in an amp. Even amping your factory speakers are going to be a HUGE improvement. Also, realize that you're only get mids in terms of frequency. For better quality, look into throwing a set of tweets on each side of the windshield and a compact sub for the back