Mine is a short wheelbase D3 W12 with 6 speed auto box. I do know it has different gear ratio to the long wheelbase W12's. I think its the same box as the S8 D3. I have now had a remap done on mine and although it does not have lots of extra power it does make it a lot more responsive.
I am very happy with mine and its coming up to three years that i have owned it. I would say it would be a very good choice for anybody.
There is however a big downside to owning one of these cars and it's not the servicing cost or repair cost because that is no different to any other A8.The big downside is fuel consumption. In the real world i get about 13 mpg round town and about 20 mpg on a run.This does not bother me as i do not drive it every day and when i nip to the local shops i use my company car (free fuel).
If you asked me would i have bought it with knowing what it costs to run then i would say yes i would do it again cos it puts a smile on my face every time i drive it