Porsche exhausts, like those of most exotic cars these days (Ferrari in particular), are really not built for performance anymore. Performance of the exhaust typically takes second place to the sound it makes under various conditions. Multistage exhausts typically only change the path of the exhaust inside the muffler system, to allow a bit of noise at idle and under spirited driving while keeping the car quiet on the freeway and while cruising. I highly doubt there are any noticable performance benefits over a single stage exhaust system.