To the OP, I have to agree with some of the posters here that the BMW dealership experience is better than Audi. AofA has to step up that is for sure.
As far as discounts if you do what I say than you can get one at $500-1000 above dealer invoice, but the wait is definitely 3-5months as the S6 has much more limited production than a 550i.
Lastly some of the guys here saying that the performance is not a factor is probably the same guys who will hark the opposite if the 550i outperformed the S6. IMO buy which ever one you feel you liked best.