For some people you can only tell the difference when you hear them side by side.

There is a huge difference between the stock speakers and a decent aftermarket set. Even at medium volume. Aftermarket produce better low end and high end ranges where stock will sound pretty washed together.

If you have the chance, go to a place that specializes in car audio. They should have a stock set of speakers on the wall that you can listen to. Then compare that to something from Apline, Rockford or even JVC.

Not sony though. Sony < Stock

Also if you replace the HU, it will put out more power than the stock HU, in which case the stock speakers will probably blow if you turn it up too high.