I did the test and evaluation of a tuned exhaust for a Ferrari 550 product (cats, x-pipe and silencers). Going back to OEM silencers gave me 15% mid range- compared to "performance". Yes, the cats and x-pipe made a huge difference. But with Porsche 911, the intake side is the weak link- not the exhaust. I just put IPD's new dimpled plenum and y-pipe on my 2011 TTS and it's now a pre-boost animal at the low end...and faster across the top...and I get 2mpg better mileage (carefully measured in two-way runs at all speeds regimes...65 state hwy...80+ interstate).