It is useless to make these broad sweeping generalisation on discounts. So perhaps a discussion on what factors drive discounting is more relevant.
Brand - branding support is critical, since if a brand is not well supported, AD's are likely to dump (i.e. reduce their mark-up to move and let me tell you that those mark-ups are huge!)
Models - all brands have a portfolio of models, some that do well and other less so. ADs discount slower moving models but retain a portfolio margin by not discounting the iconic ones.
Materials - precious metal and steel models have different price points and margins. ADs tend to discount two tone models more readily since they are like the bastard child, neither here nor there but priced high. Also the margins on the precious metal models are higher, and hence more prevelant to discounting if the AD needs to move stock.
Timing - Most ADs actually buy an annual stock of models, which means that if a certain piece is not moving by the time new stock is expected to arrive, then discounting will occur. Also, there is a timing on the exchange rates used by the AD on when he bought the stock and versus when he is selling it since he is able to arbitrage on the exchange rate to give a customer a better deal if its to his favour.
In summary, it is possible to find discounts from ADs on Pateks and AP but without knowing the above four information, it is useless to say that ALL ADs ARE DISCOUNTING PATEKS AND APs AT XX%. Also for certain brands, they will crack down hard on an AD that discounts too much. I know several ADs that have loss Patek concessions due to "over aggressive" pricing behaviours.