We have never personally had a problem getting a taxi at the port and pre-booking a car/driver will cost you more money. If you have a real early flight (before 10) and are going to worry, than you might feel better booking a private transfer (and hope they show up).
That's my concern, and I think you may have alleviated it. We are about to book an 11:00 am flight for a Sunday. Ship is "scheduled" to get in at 5:00 am, and NCL suggests a minimum of 6 hours when booking outbound flights. Still on the fence about this one... other flights give us very short connection windows.