Thanks for the info.. I will definitely be upgrading and this could be one of the routes I might take.
To upgrade the door and rear speakers, about $295.
Polk Audio DB5251 = $110
Polk Audio MM571 = $175
Metra 6X8 Adapter = $10
To do the full sound system upgrade that I am doing will cost a total of about $1,100. Along with the above speakers, I am adding in this as well....
Polk Audio MM1240 12" subwoofer = $160
Alpine MRX V70 amp = $280
4 gauge wiring kit = $75
50 foot 14 gauge speaker wire = $10
Misc. Supplies (tape, rubber washers, velcro) = $25
MTX Thunderform FMUST05 Subwoofer enclosure = $255
That includes the MTX enclosure, which is very expensive. You could get a cheaper enclosure for less than half the price that wouldn't look quite as nice but would sound just as good. Those prices also include shipping.
Once it's done it will both be louder, and sound better than the Shaker 1000 systems.