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The catalytic converters are in the headers on most BMW, your car included. Which means that even adding a full cat/header back exhaust system you will not have to worry about not passing emissions. The only thing you would need to worry about is ticket happy cops that might ding you for "too loud" exhaust. But, I've only known that to happen to JDM cars. Now, if you changed out the stock headers for aftermarket ones and did not install post header cats then you will no longer pass emissions and you definitely smell it. As for the sensors they are all plugged into the headers so once again nothing to worry about.
In short, DO IT! The Eisenmann exhaust systems sound amazing, might I recommend the RACE tone =] and maybe even the Eisenmann Quads ?
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