Unless the head unit you are getting it oddly shaped, it *should* fit in the stock shroud like a stock unit does....a 2-din/double-din opening is an industry standard size. The Impreza factory units are easily swapped with after-market units....this is not like a GM or Ford factory unit that requires a bunch of stuff to make an aftermarket unit fit and look decent. Generally, the only reason to change the bezel is either
A) you want to change the color to the JDM one
B) you broke the stock one
C) you want to add the one with gauge pods in the vents.