I'm going to disagree here. WN does exactly this, basically, and the boarding there is considerably more efficient that an AA aircraft of similar size. I can't even tell you why....just seems that people are more considerate about getting out of the aisle to let people pass...but it has been my fairly uniform experience.
Cheers.
The boarding group has more to do with window, middle or aisle than position in the airplane.
SW boarding is more efficient because people are not looking for their row, they're diving for the best seat they can find. This alone improves SW's turn time and is the main reason they do not want to give out assigned seats.