It's like that thread about the guy who ran simulations to figure out the optimal way to board planes. Even if it's true, not gonna happen, 1st class pay partly for the right to board first. Exactly the same type of situation.
First class on in what-ever order is most efficient for that part of the cabin, then the rest on in what-ever order is best for those areas.
Though what would really happen would be "first class on in what-ever order they fight for amongst themselves because they all consider themselves too important to listen to instructions from the boarding staff" followed by "the rest on in what-ever order they fight out as they don't care about the common good if that means they are on 12th instead of 1st".
In fact, I've never figured out why first class has to board first. When I rode on Cathay Pacific on business, the attendants come get you from the traveler's lounge last, once everyone else has boarded. You don't need to jockey for overhead luggage space, so why wait longer in the cramped confines of the cabin? I wish more airlines did that.