I dealt with the same issues when I was buying my S4....I was even considering an aftermarket warranty. If you follow the maintenance and don't start screwing around with the car, you should be fine.
Not to mention, the S8 has a Lambo-designed engine, so you're not going to get much of a reliability read from his experience.
Its not the powertrain I worry about, its the rest of the car... electronics, gadgets, etc.
I still might consider a 4.2 A6 is I can find a good deal on one under factory warranty still.