So yesterday I pulled the trigger and purchased top of line Eisenmann Exhaust for my 530i.
Its custom made in Germany, so I will receive it in about 4 to 5 weeks. I got the race version so it will be about 30dB + on high revs.
Total cost was $1430.00 including install and tuning.
Check the products at: www.eisenmann-technik.de
Here is how it would look like when it comes: