TomSilver:
At least in Germany you immediately loose the guarantee if you mount an OEM exhaust. Would not be an option for me with a 200k car. Even Porsches can have sudden engines failures. My red 991 had a massive engine fail at 15.000km and got a new engine for free. Imagine that with a turbo with an OEM exhaust. Brrrrr...
It is quite difficult for Porsche to prove you had a different exhaust IF the exhaust is removed before the repairs/diagnosis starts. This only works if your Porsche dealer actually installed the exhaust.
A huge problem can arise if your car breaks down far away from your dealer and next to another Porsche dealership. I would actually use the ADAC and tow the car home to my own dealership. Just as a hint... Or I would talk to that specific dealer about the exhaust and hope that he is understanding (wouldn't be 100% sure about this though).
I would also use only high quality exhaust systems...just in case, so Porsche has no arguments when it comes to a lawsuit (they usually have to prove that the damage has been directly caused or "influenced" by the exhaust).
Speaking of McLaren and a sport exhaust on a turbo charged car: Since McLaren is a small manufacturer, they have more lenience regarding sound levels and legal certification. Porsche unfortunately does not.
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RC (Germany) - Rennteam Editor Porsche 991 Turbo S (Oct. 5th 2013), Cayenne GTS (958), BMW X3 35d (2013)