i guess it is kinda true that exchange rates in korea is better than singapore. i changed at 865 at people's park a few days prior to my trip. glad that i only changed a few hundreds. went to the money changer in myeongdong and the rate was 873 (same rate although i went on two different days). but rates of money changers in dongdaemun is really bad. saw one 850 and one 860.
if you're going on f&e, i'd recommend that you change a little more than sufficient for you to get from airport to hotel and change more in myeongdong.