Security lines and immigration are MUCH better now at Incheon in both terminals since T2 opened. I would get there at 7 AM. Besides, the airport doesn't really open that early anyway. If you arrive at like 6, minimal lines will be open (and clogged as such) and most duty free stores won't get going THAT early.
But 9 AM is peak departure time for USA flights and regional ones too, so all the stores will be open around 7-8.
One thing I ALWAYS do - download Lotte Duty Free or Shinsegae Duty Free App and just pick it up at the airport. Muuuuch easier and they have good discounts. As an American, I can say that things like cosmetics AFTER DUTY FREE discounts are the same price as home. I always compare on Amazon haha.