Sounds like you've got a dilemma there Chris, or just keep both
the w12 has a load more hp however I'm guessing here the weight and less torque put it on par roughly performance wise with the 4.2tdi, it's funny how these figures work.
I watched a video of someone recently getting the backboxes off their w12, very nice indeed and sounds unique, not many w12's flying about. again another Audi that was 'muted' to some degree set it free!