I think it's a better idea than the Cayenne. Besides, recently cars like the M5, and on the cheaper end the WRX and Evo, have proved to people that sedans can be all-out performance machines.
Put it this way, if it's got over 400 hp, does .90 or better on the skidpad, and gets over 180 mph why is that not a Porsche?
Although I do hope that Porsche try to make this car the pure performance leader. The M5 will be difficult to catch with anything under 500 hp and Porsche will need to work hard to match that pace.