I think you need to deal directly with Intuit on that one.  Will Intuit allow you to transmit on the 15th and then sit on it for a day before they transmit it so it has a January 15th time stamp?  Or will it just sit on your computer and go nowhere on the 15th?  Personally, I wouldn't take a chance.  If a return is due the 15th, extend.  If it represented the extended due date, paper file.