Daz, I know of a couple of people over on Tyresmoke who've got chipped 2.5TDi Quattro's.
I think one was an ABt conversion (ECU sent away to Germany prior to delivery of the car) and the other Upsolute.
Both guys reported a good increase in mid-range performance, although it apparently made no noticeable difference to the sub-1500rpm action, which is where it most needs it - certainly with the manual box.
I was considering having it done, but given that the thing I hate most is having to work the gears to keep the revs up, I'm ditching the car in favour of a Golf R32 for now.