Having worked on a lot of permutations for Stereo upgrades, I concur with the earlier posts that you need to get 8" woofers.
You can upgrade the door speakers to coaxials if you have to from a savings POV, but it's a shame to waste the on-axis door tweeter.
You need some means of correcting the signal. Doesn't have to be recoded. Is this for the 5 in your sig?
musicar northwest portland or musicarnw.com
BMW systems and subwoofers