Yeah, I have an HKS. It's not louder, per say, but most likely has a tad bit of a deeper rumble to it.
I dunno about P5s, but the HKS is close to, if not the loudest exhaust they make for the 3, aside from the fact I'm not turbo'd or anything so I don't have a full downpipe and whatnot to add to the loudness of the exhaust, it is a full exhaust, not just a catback.
It's loud, and the intake I have adds to a bit of the throaty low tones on the lower end of the revs, but it's not a full out loud exhaust.
As I said, the exhaust on her car is lovely. A nice quite rumble and a slight reverb when it's accelerating sort of like it's looking for a nice tone to settle into. It's really a lullaby to drive to.
Glad you like it, babe. <3