I would put the Cayenne in the same category as the RRS in terms of reliablility. Too many computers in that thing!

4.8is does sound like it would fit the bill for you, but I do want to say that I still love the 3.0i the most. If you're looking for sheer straight line performance and ground shaking exhaust note, the 4.8is is the winner.
but if you want reliablility, good mpg, and great handling I would suggest the 3.0i (sport package)