If you are looking for pace and with a view towards economy - settled 4.2TDi is going to be hard to beat.
If you want the spectacle of a high revving petrol engine then no amount of tuning of your TDi will get that!
I've had a few 4.2TDi engined cars over the years. Currently, I have a Q7 (3l 272ps) and a b7 RS4, a V8 with a manual box that revs and pulls to 8250RPM. Point to point, I'd say the Q7 would be faster and more comfortable, but the RS4 leaves me with a grin I will never get from a diesel!
Your thread title was V10 itch, how bad is it, does it need scratching? If so an S6 or S8 is a good answer
I'd say the 8 is always going to be more luxurious, a slightly nicer place to be - but the 6 is very close if you need the practicality of Avant obviously wins. At the age of the cars now, I'd say buy on history and condition, you won't regret either when you find that grin.... you may when you realise how long you spend at the fuel pumps though!