Genuine rolex sellers operate like a cartel here in HK and you would not get any more than a 10% discount. The official line is that rolex controls the max discount they are alloed to offer. If any delaer offers 30%-40% be very suspicious as you're likley getting a fake or one with fake parts.
Truth be told HK is no longer the place for such purchases, especially with the weak USD. Prices of rolex watches have just gone up to compensate for the exchange rate drop.
DXB is a much better option for such a purchase.