If you haven't heard back by January 23, 2014 on the adjudication of your I-539 Application to Extend Status you are still "in status" (not an overstay) until you are notified as to the decision since you "timely filed" for the requested extension while still in valid B-2 non-immigrant visa status. However, should your extension request be denied you should be prepared to depart immediately and consider returning on VWP or on a new B-2 entry. You are correct in that if you depart during the pendency of the extension application your application will be deemed "administratively abandoned" and the extension will be automatically denied.
Answered on Dec 19th, 2013 at 4:21 PM