I believe you can get the 550 Diffuser(which has a bigger opening)and you won't need to cut anything(all depending of course on the size of the tips you get). anything over 4" would look ridiculous on a stock 5 series. Also you can get the m5 diffuser but then you will need to modify the mufflers and possibly move them back as the tips will stick out to much. Do keep in mind the length of the tips, as most aftermarket tips are way longer than factory.
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