I like to use a larger than required ESC. Say, if you are pulling over 20 AMP's on a 25Amp ESC, I'd go to a 35 Amp ESC. They stay cooler. Right now I have an Ultra Stik 25E with an AXI 2820 motor pulling a 12x6E prop. It pulls over 20 AMP's. My ESC is a Castle Phoenix 35. After a flight, the motor is cool and so is the ESC. I know that you're talking about ducted fans and the discharge is high, but I think this is still valid. You're also right about cooling. Get some airflow over that ESC.