Full range speakers mean they produce sound from low frequencies to high frequencies, and not very well sorry to say. (If you've ever heard a well set up car audio system you'd understand what i mean.)
Upgrading speakers will be a plus, specifically coaxial speakers with built in cross-overs.......don't ask it would be a long discussion.......they will sound WAY better!
If you like your stock head unit, and it does all you want, by all means don't change it! Most car audio stores sell a bunch of un-necessary gear for the average person. You will be very happy with new / better quality speakers!
Thanks so much! I will definitely be doing this in the next few weeks!