I have tried replacing the speakers on the dash, apparently the actual speaker it self is mounted along to the grill, which means If I replace the speakers, it would fall right through without anything holding it in place. To replace it, you would have to modify the mount to hold your new speakers with the factory grill.
Try to unscrew the grill, and you will know what I mean...
Not an easy task, Porsche always make things complicated.....