As the poster above said, your arrival/departure times are some of the busiest at ICN.
You will probably have at least a 15-20 minute wait for immigration to leave ICN, and even though you wont be picking up your luggage, you still might have a 5 minute queue to clear customs.
When going back through security and immigration ~8am, ICN can be very busy with all the Chinese and Japanese tour groups. Expect at least 30 minutes to get through security and immigration before getting back into the terminal. No special lines for first/business passengers either.