This is a VERY good question. To answer this, a very solid understanding of airfoil design is needed. On the other had, given the weight of an aircraft, determining the amount of power required is pretty easy.
Here is one resource that is helpful for sizing a motor for a given weight:
http://www.wattflyer.com/forums/showthread.php?t=17914
If you are starting from scratch and desire to build as efficient an airplane as possible for a given purpose, things are infinitely more complicated. For the purpose of RC, most people have an idea of what they want the airplane to look like, determine the weight based on construction technique and materials, and then size the motor accordingly.
Your question is far more complicated. I'm guessing from your question that you at least want to carry a GoPro. The GoPro website lists the White Addition at 2.6 ounces (73.7 g). I don't know if that includes the case or attachments. So, from an efficiency standpoint, what is a good configuration? Ooph! Get out your computational fluid dynamics software!
