I did the entire Shelby Kicker upgrade to my factory Shaker500 system and honestly, even with the upgraded amp to the Kicker 8" door woofers I still think the factory system provided more audible sound from the 8" factory paper woofers.
That's not saying that the Shelby upgrade wasn't better, it's a night and day difference compared to the factory Shaker500 but I think that most of the bass lost in the new Kicker door subs has been replaced by the trunk mounted 10".
The sound from the stock 8" is boomy and muddy but provides a much noticeable sound. The sound from the Shelby Kicker 8" is not as noticeable but much more tight and clean, however the 10" trunk sub really fills out where the new 8" is lacking.
I forget the specs off of the top of my head but the Shelby upgrade provides more watts to the 8" than factory (but the Kicker 8" require more) and I still don't think the Shelby/Kicker amps are getting enough power to the Kickers.
So, to summarize like a few have already mentioned...don't upgrade to the Kicker 8" door woofers unless you upgrade the power to them as well. Although they will provide a cleaner sound than stock, you will notice a big decrease in sound volume. The 5x7 door speakers will give you more of an improvement in sound than the door woofers.