There was a twin turbo 2.5 tdi in slovakia some time ago- i think it was a profituning creation- with opel turbochargers, about 400 Bhp race tune (300-350Bhp reliable). I had one with a 3L block,psi tuning chip and a hybrid turbo a few years ago- 254bhp (i still have the engine in a FWD a6 saloon allthough detuned: stock 180Bhp turbo and no chip or injectors-still spinns the wheels in 3rd gear !), i am driving now a mildly tuned a4 quattro with a 180Bhp AKE engine,rs4 rear diff,ATE 4 pots brakes/rs4 rear brakes, iveco nozzles and a PSI chip, about 220-240 Bhp. Working to fit a KKK turbo off the 3L tdi engine and looking for Bosio race 683 nozzles as well ! I also have a a6 tdi quattro on wich i have rebuilt the engine twice (first due to worn cams, 2nd time due to cheappo Dayco cambelt snapping and wrecking the engine). Now the AKE is boasting mildly polished Brand New heads, thicker 3 notches head gaskets, Steinbauer P-Box, the injector pump has been rebuilt and i am fitting a GT2559v off a renault 3L dci. This is me tow car and is regularly towing a 3.5 T car trailer. The a4 dances circles round the a6 though as its 200Kg lighter. The thing is the v6 tdi isnt that loved in uk (grease monkeys find it difficult to work on ?!) and there isnt much stuff of the shelf. Most tunners say the injector pump is the limiting factor on the v6- i beg to differ- there are cummins trucks who run modded VP44 pumps wich can supply fuel up to 1000Bhp. Also i know that the same pump has been used on Jcb and MAN trucks (allthough they have a 24v ECU) so these should supply decent fuel to a hungry 6L v6/v8 engine ! The 24v are also rarely failing unlike the 12v Bosch computers ! There are no aftermarket cams for it and the audi heads are overengineered so there isn't much to improve on. The only thing i would upgrade is the inlet manifold but isnt easy to do as the injectors hard lines are running over it. Fit a bigger FMIC and you will be good to go !