Hey champ and welcome..
Unsure really how loud it would be, but it would have a bit of a note on it..
Best way is to ring and ask an exhaust place, these guys would know the approx sound ranges it would be pushing out
It's weird.. but even if someone has the same car, it always sounds diffrent, let alone diffrent loudness.
It also comes down to the type of exhaust you want... I have hurd and seen thinner exhausts tend to have a tinny sound. My exhaust is custom and thick on my rs subie, and she is quite deep and loud.
So just give an exhaust place, that you know of that does Subarus, it would be alot more helpfull..
Maybe someone here could also help, but best would be an Exhaust place
Cheers
Mano