The A8 D2 3,3 V8 TDI is an awesome car, mapped ones are quicker than S8's. 70 mpg is a pipe-dream though, regardless of mapping techniques.
In terms of torque, I believe the ZF trans will be the limiting factor here. The motor is capable of a lot more than Audi claim, but torque is toned down to keep the tranny happy. Stock S8 trans are not as long-lived as the same in an A8, not because it's weaker, but because the S8 produces more torque. The 3,3 TDI V8 produces even more, with a massive hike in torque, it won't be long before things start to break. No manual option either.
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