Korea exchange rates have always been fluctuating but I've not seen rates in SG better than Myeongdong, yet. Most of the rates in Myeongdong are similar due to competition(except those very small stalls, got lowballed once), only the one at Chinese embassy is slightly better. I've seen people running up and down comparing rates and probably spent at least an hour doing that. In the end, they changed less than $1000

The difference of KRW1000(about SGD1.20) is not significant and simply not worth the effort. They could've spent the time shopping or doing something more enjoyable during their holiday.
Not that I'm against looking for the best deal, but during holiday your time is precious. Why waste so much time and energy just to save SGD1.20? Can't even take a train ride in Seoul. Just compare a few moneychangers, make sure you're not getting chopped and get on with it.
For those looking for the Chinese embassy moneychanger:
Come out of Myeongdong Station(migliore exit) and walk straight. When you see Nature Republic(green building), make a left. Walk straight all the way until you see a carpark. Turn right and walk straight(30-40m?). The popular moneychanger will be on your left. There's usually a small queue so you won't miss it.
There's also a moneychanger at Itaewon giving pretty good rates. However, I've never gotten the chance to compare these 2 moneychangers because they don't publish their rates online. For those changing bigger amounts like more than SGD15,000 you can consider going to the banks. Can negotiate and bargain with the RM

I changed $1000 at Bank A and brought the receipt to Bank B and bargained with the RM for the rest of the cash. Got quite a good deal ^^