Hartge has just released some stainless steel exhaust systems for the BMW M3 E90 and E92. These systems improve the sound the looks but also the performance up to 20hp according to Hartge.

M3 (E92) Coupe:

18 92 0500 G
HARTGE stainless steel exhaust system with sports catalysts (export version) with dual round tail pipes left and right, performance increase of 20 hp
18 92 0550 G
HARTGE stainless steel exhaust system with sports catalysts (export version / sound +) with dual round tail pipes left and right, performance increase of 20 hp

18 92 0400 F
HARTGE stainless steel rear silencer with dual round tail pipes left and right

18 92 0450 F

M3 (E90) Limousine:

18 90 0500 G
HARTGE stainless steel exhaust system with sports catalysts (export version) with dual round tail pipes left and right, performance increase of 20 hp

18 90 0550 G
HARTGE stainless steel exhaust system with sports catalysts (export version / sound +) with dual round tail pipes left and right, performance increase of 20 hp

18 90 0400 F
HARTGE stainless steel rear silencer with dual round tail pipes left and right

18 90 0450 F
HARTGE stainless steel rear silencer (export version / sound +) with dual round tail pipes left and right

You can buy Hartge parts here.