The n55 is one of the best sounding motors imo. If you are just going for sound a downpipe may be enough. Downpipe and bmw performance exhaust is just beautiful.
Here is my car with dp and bmw pe exhaust. Second race you can hear downpipe and stock exhaust from my buddies car. Both cars with pure stage 2 turbo and injen intake. But stock turbo will sound similar. So good.
https://youtu.be/lgHW_RcHsqA?si=obhXLvzoqwg8mVo2