Originally Posted by FASTBEN
and yes that motor is under rated.
Just because you're getting away with it, doesn't mean the motor is under-rated. You are running a duty cycle of around 1%. In other applications that need high burst power (eg. F5F hotliner glider, 3D aerobatic plane) the duty cycle might be 10% or higher. That would create 10 times greater temperature rise, and probably a cooked motor!
Maximum power transfer is achieved at 50% efficiency (which means that half the power is being wasted). That is how you get the absolute highest possible power output, but backing the loading off a bit won't reduce it by much. You could drop down to a 14x9 prop and get almost the same climb rate, but with less risk of burning up your ESC.