I have a few interesting chats with dealers over the last week (not VW) - and been chucking a few broker prices around at them. These discounts that are being used by brokers aren't allways what they seem. They may well include any dealer contributions already being offered, so that changes the discount percentage. Additionally, the broker has to actually find a dealer that will agree to that discount - they may have had one (hence they can advertise that figure) but that doesn't mean you're going to get it.
I'd eat my own hat if you could get 15% of an R right now - given the fact that there is a 7 month odd wait to get them built, and clearly still a desirable car, why would dealers give that kind of discount knowing full well the next person that walks into a showroom may be very happy with 8%? I don't buy it.
Broker prices are a very good starting point for haggling with a dealer, but 15% off an R - no chance.