It sounds like you're on the right track but if you go to any audio convention or get an audio guy in your car you'll probably get criticized. Personally i'd upgrade all the speakers even the rear. If you get good enough tweeters they're sound awesome and you'll have no need for a pair of speakers that are a comp set (with a tweeter in the speaker) You'll have so much high freq notes in there it might sound annoying.
Upgrade your front or rear speakers to a pair that are built for midrange.. You don't want all high note in the car and low note from the sub then you're missing the most important part which is the midrange.
Low - Subwoofer
Mid - Mid range speakers
High - Tweeters/comp set of speakers
I can't really put it into words...
Read that link, it goes over mid, high, and low.
Now as far as the rear speakers go, I really would replace them, It's a good idea.. I know it's a bit of a hassle but it's worth it, although you really don't hear them that much, they do play a key role in the process.