Just spent almost two weeks in Korea. I'd advise anyone who's going, change only the minimal possible in SG.
There's really no point changing in SG when MCs in Seoul are offering rates unbelievably high, you won't believe it until you visit them yourselves.
Last week, when arcade was offering 858 and then 862, I got mine changed in Seoul for 870, then 875 last Thursday. (I can show the receipts if needed)
This M.C in Myeong Dong is ALWAYS offering higher than what the best in SG can offer, so I really don't understand why people STILL change big amounts here in SG, it's really losing money instantaneously by doing so, especially for those who change by thousands at Arcade and still feeling happy over it lol.