I wouldn't worry about it. 2 1/2" to 3" will get your suspended ceiling in with a tight fit and your ok anyway. Just build to fit your basement and don't worry about an inch or two. The codes work for new basements, but many were built in this country with only 7' walls or less to start with. You can't make the code retroactive. Kind of like egress windows. Code requires them if you want to label a basement room a bedroom. But they can't stop someone from sleeping in their basement if they want to without one nor can they make you put one if just cause you sleep down there.