I'm not familiar with the dimensions of that particular aftermarket head unit (nor should I be, since I'm not the one buying it), but I'm pretty sure the dash kits (sold separately) are customized to the stock vehicle bezel and provide adjustments for that bezel.
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.
2008-up Impreza and 2009-up Forester OEM HUs are non-standard size. Unlike previous years, the opening is larger than standard double-DIN. So when you install an aftermarket HU, there will be 1/2" gaps on the left and right sides.
See this post. For 2012 Impreza (not WRX or STI) see additional notes and link in that post.