What did you end up doing in the end?
I've got a spec C, so it's only got the two front speakers - no tweeters (front) or rear speakers at all. The speakers in the front are like the "cheap black plastic cones" you describe - the speaker is integrated into the mount, so they're junk really.
From what I can tell, there seems to have been a premium option, that includes the tweeters and different speakers/mounts in the doors. Some details here:
https://www.crutchfield.com/S-Kg6UDS4ZgaK/learn/2008-11-subaru-impreza.html http://www.clubwrx.net/forums/electronics-car-audio/134347268-guide-replacing-door-speakers-08-wrx-impreza-spacers-adapters-instructions.html
I called around some wreckers to see if they could get the tweeters and "premium" drivers (with integrated tweeter), but it seemed to be too hard. I think eventually I'll just go with some custom mounts and aftermarket speakers. Could order all the premium parts from jp-carparts (or similar), but I think that'll be more expensive than just going aftermarket.