I have a 2009 Carrera S and with 27,000 miles on it the tensioner bolt suddenly sheared off in July (2016). My mechanic replaced the tensioner, belt and bolt but could not determine why it had happened. All pulleys spun freely. Last week, December, 3000 miles, later the bolt sheared off again! He confessed that he had used a standard bolt the first time (I was rushing him) and perhaps it was a hardening issue so this time he used an OE bolt from Porsche.
Not a comforting situation. I have owned many Porsches and never had this occur although its better than the time the (996) engine