Audi A8 D3 diesel tuning potential?

[Bushido]

Hi all,

I cant seem to find a better section to attract the attention of those in the know, so will give in here a go!

My d2 s8 is awaiting its box rebuild (yes, still...) I have a winter hack of a d2 2.8 QS which is a reasonable daily runner. But the fuel consumption barely warrants the lack of poke it genuinely has compared to the s8 and tuned Saabs I usually drive. They will typically return 25 + mpg with 275 bhp as a petrol turbo. The 2.8 QS offers low 20's without the fun or adrenelin whoosh...

So, long story short - with the D3 diesel range -

Which model responds best to a remap?
Which of the engines and boxes are most favoured as the one to own?
Would prefer keep a quattro system as its quite reassuring to know its there.

Would only be looking at a 4k budget which might make this whole thought process null and void when I start to look more carefully online. Seen a few at £3.5 - 4k but dont know just how good such a car could prove to be?

Genuinely welcome any real life thoughts and experiences folks!