The suggestions above being more reasonable and right on target, I'd like to also suggest you'd have plenty of time to get outside of security and back in. The longest walk from the end of the concourse to curbside is about 5-8 minutes. Clearing security, even with long lines and not utilizing the First Class lane, has never taken me more than 10minutes and generally takes about 90 seconds. Unless I'm misreading the goals above, I think you'd be able to walk briskly, get your stuff, and get back in even if boarding had already started..