Wempe, Huber and most official Patek agents in Germany won't provide you discount more than 10%.
Like many other brands, JLC has, in general, much higher discount rate than Patek.
When I bought my JLC, I was offered 20% off. Later I know that I could easily ask for 25%. In HK, JLCs have even higher doscount.
Different brands have different channels and policies, direct comparison makes no sense. Furthermore, different models have different discount rates, even they share a same brand.
In general, gold models have bigger discount than steel models. This holds true for most watches in the JLC league. Of course, it doesn't necessarily to be true, when it applies to Patek Philippe or Lange & Soehne, since seldom do they produce steel models.
You could expect about 12% tax back if you hold short-term visiting visa before Jan 1, 2007. Now since Germans increase VAT to 19%, I guess you can have more back, something around 14-15%. However, you also pay more than earlier.