I think it's important to do your homework on the specific model of Rolex you want before you go overseas. It's a case of supply and demand.
my partner found out that the Rolex Milgauss is cheaper in HKG but the Daytona is more expensive in HKG, both compared to SG. It could be because they prefer the Daytona there.
You can ask Hour Glass or Sincere if you are good terms with them, to put the item their airport outlet. Pick it up before you go overseas to get the 7% GST exemption. But must be non-Singaporean passport.