As promised, here is the straight and narrow on putting in aftermarket speakers, at least for a WRX wagon, WITHOUT having to modify the door panels. I'd expect the sedan to be pretty much the same; perhaps someone can give input one way or the other. I have fotos and drawings that I can put on my server if anyone is interested.
I'm pretty sure I hit every auto sound shop in town over the weekend. There just doesn't appear to be a "bolt-in" replacement for the front speakers. Even the Fosgate driver referenced in an earlier post will require the installer to fabricate an adaptor plate. Altho there is adequate space in the tweeter pod, plan on building a custom bracket here as well. There is lots of room in the back, tho; most any 4in aftermarket will fit into the stock "adaptor", and I suspect that anyone wishing to do so will be able to fit a 5&1/4 coaxial without too much bother.
Using my special and highly technical "internal clearance gauge set" (modeling clay and a steel ruler) I measured clearances around the speaker mounting locations on the passenger side doors (I'm assuming symmetry left to right; gulp ;-). There is 1& 9/16in of clearance behind the stock mounting points before you are into one of the mounting screws for the window. There is 5/8in clearance in front of the stock mounting at the closest point (the top screw) before you run into plastic.
That doesn't leave much room for noisemakers, does it?
After weighing the options, and measuring several "shallow" aftermarket drivers, here is what I decided to do:
The 5&1/4in Infinity Kappas have a 2in depth. These will clear the window hardware if mounted on a 1/2in thick adaptor plate (trace the "triangular" stock speaker onto 1/2in thick material (I plan to use acrylic ((it's THERE in the junkbox)), but plywood would be an obvious choice), cut out, drill for stock mounting holes, mark and cut 5&1/8in cutout for the Infinity speaker, pre-drill mounting holes, etc.). If you take a close look at the Infinity speakers, you will notice that the rolled surround actually protrudes from the front a little. By my calculations, this will just miss touching the inside of the plastic, but I plan to trim a couple of the high spots just to be on the safe side. The tweeter will mount in the stock location on an appropriately shaped and drilled strip of sheet metal. The (rather large) Infinity cross-over will have to be mounted somewhere as well, but I haven't addressed THAT issue yet. Some older 4in Alpine speakers will fit in the stock mounting in the back. (will probably go to Infinitys later, but I already had the Alpines)
Someone had mentioned using the 6 1/2in Kappas. Probably not unless you want to modify the plastic; those surrounds are gonna hit otherwise. Of course, if you don't mind heating and reforming the plastic, cutting the door panels, trying to match an aftermarket grille to the graceful curves of the panel, etc. you can probably fit pretty much whatever you want with an appropriate adaptor.
All of this is still somewhat hypothetical; darkness and snow overtook me yesterday, and it's awfully cold/snowy/windy today with more on the way. As soon as it warms up enuf that the tools don't stick to my hands I'll finish things up and let you know how it went.