Yeah 16 minutes from Metro arrival to boarding is maybe a bit too close. You're relying on Metro to be very on time at that point. But it'd be interesting to see how long it actually takes you. My experience with the early TSA line at DCA is smooth even in the non-precheck line, and none of the gates are a long walk.

Edit: and especially since your plane isn't coming from somewhere else, there's no turnaround time for the plane to be cleaned and refueled. It's a high likelihood of leaving on time or even earlier.