sound quality will NOT improve with new speakers in the rear. The rear speakers are very unusual and are almost as good as their gonna get, unless you wanna rewire, set up an amp and face unit and some other stuff. Dont mess with the rear end of story. Buy a sub and some nice front and maybe a HU and u should be set.
The rear speakers on the base system have the highs and lows cut, and the midrange boosted. Replacing the speakers alone is not going to give you too much in terms of improvement. Crap in Crap out. You can't polish a turd. If you want your rear speakers to sound better, you'll have to get an amp, run it off of the output from the front speakers that is much more flat, and run it to the rear speakers. See this thread for reference: