The A6 came with various TDI engines,5cyl,V6 and 4cyl. None of the turbos' are a direct fit the most straightforward will be from the 4cyl but it will be a VNT type and your ECU from the TDI conversion will not control the VNT unless you have an alluminium case'd ecu which can be switched to control vnt.The european mk3 golf came with 90 or 110 bhp option and these alluminium cased ecus' have the VNT programme installed but are deactivated in the UK (only 90bhp in uk and no VNT).A clever remapper can enable the programme and away you go.The plactic housed ecus' dont have the vnt programme installed so its a no goer!
You can use a Passat AFN alloy cased ecu for VNT control on a Golf loom but there are a few pin changes to be made depending on what year loom you are running. VNT is the way forward much better choice of turbo's with much more response and efficiency. I am now running a GTB2056vl which is big enough to support 200-250bhp in a 4 cylinder application.
Different N75 valve and software and of course making a manifold or adapter to fit your choice of turbo, the 2.5 109LT's run VNT turbos so the files are there for the 5 cylinder stuff in standard boost form but as for fitting one never tried on a 5 cylinder in a T4 I haven't found anyone with enough money to let me play, i will always be a 4 cylinder boy so the chances of me ever building a VNT setup for a 2.5 are very slim, it is going to come down to finding good mappers that are UK based if you are looking for big numbers (same problem different setup) !!