FR-Rush - not quite true, i'm afraid.
Sports models get a 6 1/2" speaker in the front door, and a 1" tweeter in the dash bonded into a clip-in mounting ring. Lower spec models got a full range speaker in the dash, being either a 3 1/2" or 4" size, I forget which. It's NOT a standard fitting, so any replacement will be a custom fit (or require a custom mounting ring). Sony do (or certainly used to do) a custom fit 2-way, not sure about other options, sorry. A 4" speaker will never shift much air though, and the location is less than ideal for a mid/bass unit as the speaker fires directly upwards at the screen - so this may be quite a pricey upgrade for not much benefit.