Yes, I have upgraded my OEM speakers W/out the use of an aftermarket external amplifier. Was it worth it? Depends & varies user to user. It's difficult to find a set of aftermarket speakers that will sound good off of 18-22 rms or so (most HU's put ou this). Often, you will find that aftermarket speakers are spec'd at nearly twice, or even three times that amount.
I am almost guranteeing an audible notice in SQ. I am almost also guranteeing that your aftermarket speakers will clip at high-er volumes.
Was it worth it for me? Yes. I purchased individual drivers based on the specs I was given & suited accordingly. It's up to you. If you are going to take the step than I'd advise you to properly treat your doors appropriately.
"What type of music do you listen to?"
^Why does this matter? Regardless of how cone composition may sound better at "X" frequency than another, the listener may prefer a wide range of genre. Example: I listen to rap, so I am assuming you may recommend me a set of speakers W/ a good mid? When I say "good," I mean "beefy." -You catch my drift. Well, how do you I don't enjoy the robust natural presence of paper cones. I also listen to Shania Twain & Mariah. Now what are you going to recommend me?
Here, this may help: "New To 2012! The All New Universal Cone Composition specifically engineered to kick asss from Elton John to Eazy E!