Do VW actually sell any of the V10 versions though?
I can't quite see why you would want one.
For sporty types, the Cayenne turbo is the way to go (beats an Impreza WRX round Bedford Autodrome according to Evo).
For less sporty types there is the V6 version.
For more frugal and mud plugging types, I could see a market for a smaller diesel version, but not a monster one.
More of an issue would be product positioning - people had a fit when Porsche said they would do a truck, think of the fuss if they said they were going diesel.
There would be even more fuss if they bought the technology in (since unless there are up to some secret development, they have no in house diesel capability).
Besides, a rattly old bag of spanners in a washing machine, is hardly the engine note worthy of the Porsche crest, together with the cloud of carcinogenic black smoke.
Having said all this I suspect it is all just a matter of time.
More of a question is "How big should Porsche become?". What is the "correct" number of cars to sell.