Norvil do a selection of lightweight discs.
I bought a front and a rear disc from them for my MK3 four years ago. The front disc has been faultless and works well, but I have now had two rear discs, due to my first rear disc wearing out on the disc and the centre about the bobbins. My second rear disc has just worn itself out again as per the first disc. For personal reasons (I don't want to libel the site) I have not bothered contacting him about the second failure, as I am sure that the same disappointing response to customer satisfaction would recurr. I'm just going to screw it to the garage wall as a momento, and re-fit a standard disc on the rear.
So there you have it. Excellent experience with the front disc, and a bad experience with the rear disc. They do look nice as well in my opinion.