Depends what the muffler is like, if it is a massive cannon of a thing with like a 6 inch straight through design then it might make it super loud, BUT since it is a turbo and it is only a muffler I doubt it would get loud enough to fail a wof.
So in short, yes it will be louder but I would be surprised if it got super loud since the rest of the system will be stock with all the factory restricters and bits and pieces that keep the sound volume in check.
Should give you a bit more of that nice subaru rumble though if that is what you\'re after
Do bare one extra thing in mind though, fitting a super massive muffler (if it is a big thing) WILL make your car 500% more likely to get pulled over by the police/harassed by boy racers.