I had a straight through 3" stainless exhaust on my e60 535d it sounded awesome. Will do something similar to my 535d GT in time. I got a local garage to fit it.
my site has details and pics, i think there is a sound clip in the links page.
www.bmw535d.co.uk
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540i
GTR-R35, F90 M5, X3MC, RS3, RS7, M5, S7, S3, RS3, RS6, GTR R35, 996 Turbo, M5, RS6 V10, Cayenne Turbo S