That's normal. It's common for the exhaust to be louder when cold. Any chance the car is parked between buildings or other cars? That might make it sound more boomy since the sound is reflecting.
Aftermarket exhaust will almost always make this worse since they're bigger/wider and the air has more volume to bounce around in.
I was actually shocked by how quiet the 11 WRX exhaust is. But it might be a comparison thing...what have you been driving until now? I was in an Evo 8 so that may explain why I think the WRX is basically silent lol.