In my Panther I'm using a Hacker B50-15L on a 6S3P LiPo pack(960 WATTs). No cooling for the Midi fan. So far no problems. I keep flight time down to 6 mins per flight. Never heard of a Midi fan melting for motor heat. Just remember to support the back of the motor in the fan.
Mine flights like a rocket. See http://www.rcgroups.com/gallery/sho...&cat=500&page=1
Bob