I had a PE exhaust installed on a 2008 328 XI and I can tell you the sound when new is NOTHING like how it ends up. New, it's barely there.
I went from being disappointed (having to free rev it inside parking garages with windows down to feel like I had an exhaust) to people telling me they heard me open it up from a half mile away when I got on the highway, to people telling me it sounded illegal (half joking of course).
But it does get substantially louder - when its broken in it really rips, no problems hearing it open up when you are inside the car with the windows up. I felt like it took 8 months or more to fully break in, but my memory is sometimes.... not so good
So anyways, not really sure about the fact that you are missing some hardware - I would get that resolved for sure. Then be prepared to give it some solid break in time.
That being said, if anyone knows why an exhaust would need to break in, I'd love to hear it. I've always felt that metal pipes shouldn't change much over time.