sorry for the late response...
in short... after getting my new speakers WITH tweeters built in... I noticed an incredible performance increase in sound over stock.
Changing out tweeters would probably not do much difference to the overall sound quality. Probably placement of the tweeters would do the most good. With one 12" kicker in the back, the tweeters built in the speakers with that of the dash as well do the trick. Before I moved from stock speakers (no tweeters) just using dash... huge crap when listening to anything with bass and high's while sub is goin off. Sounded like the high's were drowning in a pool of mud.
All fixed now
BTW my speakers are polk db 570's at the back and 675's for the front. These are NOT component sets... but I didn't need them. They do have component sets of these though... check out crutchfield.com