i've replaced the stock speakers in all my cars, except my ms6, and its completely worth it. the highs are 10x better and they dont distort so easily.
also consider adding some dynomat to the doors while you have them apart. this stuff works wonders. not only will help the speaker by providing a better enclosure for it to play in, but it quiets down road noise to.
as fair as the tweeter goes. you could just replace the woofer with with a coaxial speaker which has a tweeter in it. or replace the woofer and tweeter with a good set of components.
i'd suggest the latter, buth components really only shine with an external amp.
components+amp+dynomat=kick ass.