Thanks OP for the post - most useful.
I've been researching the heck out of this subject in a bid to do mine sometime soon.
I noticed that the diameter on yours is 310mm whereas on my 3.0 TDI (D3) the rear discs measure to be 280mm. So I ordered some Brembo ones from Amazon (didn't even know they sold brake discs!), I already have the pads from when I purchased the front and back pads, again Brembo ones.
Here's the link for the Discs:
Brembo Rear Discs
On a side note, around 3 years ago one of my rear calipers would not unbind off the disc, and it was diagnosed to be a bad caliper.. the car ended up at a German specialists and they proceeded to sting me with a £600ish bill for the replacement caliper + labour costs.
Upon collecting the car, I was informed there had been a mistake within the invoice, and there had in fact been another part which had 'gone' and thus had to be replaced, taking the total invoice to around £1200..
Having watched the Ross Tech video, and particularly paying attention to the part where it is stated that on some A8's if you don't wait the 30 seconds or so after the calibration, it could fry your EPB module, i'm now starting to wonder..