A lot of the time, people will decide to go with a big turbo after buying stock turbo software. So, in order to recoup the cost of their stock software somewhat, they will put ECUs up for sale.
Unfortunately, swapping ECUs in TTs is a bit of a tricky task, which I personally had a nightmare with. First, you have to make sure the ECUs match and then, you have to fiddle with the immobilizer.
In order to swap an ECU, the ECU to be swapped has to be from a car with the same engine code and transmission and usually has to have the exact same part number. No, a 225 ECU usually won't work on a 180 and vice versa.
To check your ECU code, you can either scan with VAG-COM or pull your ECU out. This, when I did it the first time, was zero fun. The procedure isn't that hard, but Audi made sure to really wedge the ECU in its mount very well. It took an hour of prying to pry the ECU loose. APR has the best instructions; it is step seven in this writeup that is the real pain.
The ECU code, i.e. the ECU part number, is the large series of letters next to the Audi rings above the barcode on the ECU housing. The number should read something like
"8N0-906-018 B" or similar.
A side note: I installed an ECU from the same engine code and transmission but the last letter was different. It installed fine and started my car, but immobilizer disarmed me immediately, so I'm not positive whether an ECU with a different letter at the end will work completely fine or not.
If you have questions about ECU compatibility call APR Tuning and ask for Chris; he helped me through my ordeal. APR's number is 334-502-5181.
So let's say you've found a matching ECU; unfortunately you're not in the clear yet. Most TT models have an immobilizer (apparently, some have no immo), but some from the 2001 model year onward have a more complex immobilizer. 2000 and some 2001 cars usually have Immo II, which means the ECU has a code programmed into it that must match a code programmed into your key in order for the car to run. All cars from 2001 onwards (including some 2001 cars) have Immo III, where the ECU's code must match a code programmed into the cluster and the key.
This leaves you with two options: reprogram your keys and cluster (if necessary) at the dealer for what will likely be hundreds of dollars or purchase an immobilizer defeat code. You can also try and fiddle with stuff yourself using VAG-COM but you need a key code to access the immobilizer data if you have Immo III. These days, only dealers can fetch this, and good luck getting them to do so for you. Immo II looks fairly simple, although I do not have experience doing the procedure.
Immo II swap procedure:
Immo III swap procedure:
It is NOT POSSIBLE to switch immobilizer chips on ECUs to make them compatible with the cars. I've been there, it doesn't work. End of story.
I personally wouldn't do an immobilizer defeat since it would make the car easier to steal. I like knowing that someone can't take off with my car unless they have a programmed key that matches it. On the other hand, Immobilizer defeats are also useful if you want to install a remote starter. So if you want to, APR charges $150 per ECU for an immobilizer defeat code. Revo also does immobilizer defeats although I'm not sure what their pricing is.
In the end, I really don't think the whole ECU swap idea is worth it if you have Immo III, since it will cost at least $150 more to get the immobilizer erased or possibly several hundred dollars to get keys/cluster reprogrammed. The DIY route with Immo III entirely depends on the dealer's willingness to fetch a code for you too. Immo II seems somewhat feasible.
Forking over several hundred extra and just buying software seems to be the simpler way to go.