There's a few bits on the Bitdi thread on this A6 forum. (Not sure how to link from a phone - sorry).
But to paraphrase it - 3.0tfsi is indeed rare, more so as it never worked over here from a company car tax point of view with the emissions, mpg vs the 3.0 diesels and due to this depreciation was heavy. Although as you will see comparitivley it's quite a bargain.
I also believe it runs a slightly detuned version of the S4 / S5 engine, so is easily tuneable....
I have gone for the Bitdi as have been burned by a niche car in the past - 535i active hybrid, which whilst a lovely car it sat inbetween the 535i and the 535d with only about 10 % better mpg over the petrol and a tiny tiny boot due to the battery pack. (Depreciation was horrendous!)
Bitdi is a remarkable car, am going to pick up some time later in the month (as need to sell my current wheels) - and they are starting to get down to sensible money now that the facelift is out a while.
Just my tuppence worth!