So, I assume that Derek is the one who "spec'd out" the 15-20....he was the first to show and tell this motor.
What would have you done differently, if you had spec'd one out, considering the issues at hand with powering the 15-20? Sounds to me like Derek, or whomever, went for all out power with few considerations.
Personally, I would suspect that the prop, battery, and assumed payload would be the driving force. I am not an engineer of any sort so I ask here.