I don't think they can revoke the warranty just because people don't take their cars to get serviced at authorized Porsche stealerships. If that is the case then there are a lot of Porsche owners out there with useless warranties. I've pampered my car, serviced it correctly, and it looks pristine, if they don't warranty a problem if it arises then all hell will break loose.
Sorry, but I won't pay $800 dollars or so for some visual checks and an oil change.