Hi there! Good luck shopping in Paris. I met a really nice SA there if you needed her contact details. Regarding additional discount or cashback, I can't comment on it as I've only been a visitor in France but places like Printemps and Galleries Lafayette will ought to have them.
However, tax back % is better when purchased from Flagship store rather than a mall like Galleries Lafayette. keep in mind it'll still not be the full amount rather a 12% only, not the full 20% VAT.
Feel free to correct me anyone. Also, Try to reserve the item in advance to avoid disappointment as we are approaching Christmas season and popular products may be scarce.
Thanks and good luck.