Officially, there is no holdback on BMW's
http://www.edmunds.com/advice/incent...ack/index.html

But I know that all dealers get insentives and bonuses from amount of cars sold. Additionally after the sale, you will be called by BMWNA rep for a survey where you will be asked multiple questions on how you liked your experience. If you give all 5's the dealership and the sales person gets better rating which in turn means more money for them (bonuses, etc)

So in theory if they had sold the car for invoice and you come back with all 5's they are already ahead of the game.
I highly doubt any dealer would do that, but there is no one stopping you from trying.

I know I got my very decent discount, but its not realistic to get to invoice.
Even now I get callbacks from other dealers asking me if I made a purchase and most say good luck and that they couldnt have matched the price after I tell them my discount.