I did find a couple people that had replaced the subs with 6.5's. Here is one article
http://www.renntech.org/forums/index.php?act=attach&type=post&id=5022
About half way thru he talks about how to take the door panel off and build a baffle for the larger 6.5 speaker and keeping the stock grill.
Just replacing the speakers should give you better bass. If you run full range the sound stage will still be forward of you and not really fill the space around you. Instead of front and rear you would have top and bottom. It would be interesting to try and see how you like it. I'm still thinking about how to put in some rear speakers in the 5-6 range.