while i agree its different, and that it would be streight forwards to install etc, i simply dont agree that it can be made as fast as your 1.8t, considering the 1.8T must be pushing around 280HP with that APR Stage3 turbo etc
Yes, they might be able to make 500lbft (i've never heard of it mind), but even if they do, its only because its creating a huge spike at about 1200rpm, and the torque drops off rapidly afterwards. Its a given nature of a diesel. You can kid yourself forever that lots of torque makes a car fast, but what you convienently forget is that a) diesels dont rev, so the gearing is longer, instantly diluting the torque at the wheels, b) torque is a measure of an instantaneous force, power is a measure of work done. Work done dictates how quickly you accellerate, not the torque. That means its the area under the power graph that dictates how fast the car will accellerate, and even with your massive torque figure, i very much doubt your going to get much more than 250hp peak out of a tuned 1.9, and thats with spending a LOT of cash on it, and using a PD engine to begin with.
The other issue is that most of the common tuning parts such as upgraded turbos for the 1.9 are for the transverse engine installation, meaning they wont bolt streight onto your A4 lump.
The same cash spent on your 1.8T would probably see it knocking on the door of 400hp, or you could transplant an S4 lump or a V8, perhaps with a pair of turbos, in there instead and have much more interesting results.