No offence DPM, but I would have thought the RS6 replicas he was purchasing would have an offset of ET35 or there abouts for Golf fitment, and looking at his car pic, I believe it's for a B5 Audi A4 Avant?, so it would most probably need the wheels to be ET35 offset if they are 18x8 in size. Adding a hub adaptor would most probably change the offset, so say a 10mm hub adaptor would mean the wheels would become ET25, which in my opinion would make the rims stick out too far!
I'm all for hub adaptors mate, especially for rare wheels or the chance to be unique, but I wouldn't recommend them for a set of RS6 reps, which are widely available in the correct fitment for your car!
Just keep tabs on ebay, they come up now and again at the right money, and you will have less hassle in the long run if you get ones in the proper fitments.