If you want loud, the BMW performance exhaust may disappoint. The BMW perf exhaust has a great sound but it is only slightly louder than stock. The change (and value) is more in the tone.
The berk race will be considerably louder and has a good aggressive tone to it. It is also a lot cheaper than the bmw performance exhaust. It should not violate your warranty in anyway.
MY Build Log
08 AW 135i | Cobb | ETS FMIC | ER CP w/ Tial Bov | AR Catted Downpipes | Berk Mids | Meisterschaft GT SUS 1M |M3 Control Arms | M3 Sways | M3 Rear Subframe Bushings | Wavetrac LSD | AST 4200 DA Coilovers | HRE R43 | Hankook R-S3 |Full BMW Performance Interior | Revozport 1M Raze Aero Package in Satin Carbon |1M Body Conversion|