The A8 is Audi's equivalent to the 740, except that Audi also provides Quattro all wheel drive, and the car has an aluminum space frame.
I had the use of an A8 for a week, and it was not up to 740 standards. But the S8 adds a sport suspension and 360 HP motor. This is what was missing in the A8. Audi's 4050 lb curb weight is considerably under a 7, and it provides the AWD as well. According to the article, the S8 runs the 1/4 mile in 13.9 and has a top speed of 155. And at $74,000, it is reasonably priced.
Attention, please, BMW: put the M5/Z8 engine in the 740 sport and call it an M7. EVERYBODY will want one. It will be the finest luxury sedan in the world. You are being eclipsed by Audi, and there is no reason to let this happen.