Another Polish person here (shopping in Paris)
My two cents:
If you already have a good SA, definitely keep them (also you never know how long they will keep working for Hermes).
I don't know the prices at LV, but Vitkac and Moliera2 without any sale is actually more expensive when you convert the price into euro.
I do not believe for a second that a polish boutique will recieve nearly as much bags as a Berlin one.
Also, polish zloty is cheap for foreigners (both local and turists), so it's not only our celebrities you have to worry about.
Optimally, if you can, keep your SA in Berlin and create a second relationship in Warsaw.