Wow, lotta hate for the Subaru boxer-4, haha.
I had a 2009 WRX with a catback on it a few years back and loved the sound! Granted, I had the "SPT" exhaust from Subaru themselves so it was more subdued than many other aftermarket options you'll find. If you ask me, there aren't many four-cylinder motors that sound better! I'd certainly rather have the car sound closer to Subaru's turbo H4 than what BMW has for the turbo I4 now. I've heard BMW has made some improvements though, and that's great! I haven't gotten the chance to hear these reported changes to the sound though.
The motors are actually quite tunable, lots of options out there. "Stage 2" (turboback exhaust + tune) on a regular WRX motor would put people around 300ish HP at the crank pretty easily if not at least the high 200s at a minimum, even with the stock unequal length headers.
I get it though, everyone has their own preferences, and that's fine too, haha.
I would agree with others though, please do not ever consider, much less actually go through with, putting the Subaru boxer motor (even if it's from an STI) in your Bimmer. There's a certain character to each the car and the motor and I don't think they'd be a good fit. Either are great on their own though and with their respective chassis/engines.