You don't need to go through customs in IST BUT you will go through security. I don't know if it was just because I was taking a flight to LAX from IST but I have to say that their security is INSANE. It took at least an hour just lining up and going through security check. They check your bags twice via X-Ray then do random bag checks AND then they check your passport a ton of times before you finally get to the waiting area.

It's absolute chaos.