how to keep mm 5700 cooler

If you're running the 5700 within spec, that is, not exceeding the recommended voltage, you should be able to get it to run at normal temps when you have it geared right for the vehicle, motor, voltage, tire combo that you are running. I'm running a 7700 system on 3S Lipo.

I wound up with a 12/87 gearing, punch control at 100%, startup power and timing on low. I can run all day without any heat issues.

The MM system is a good system and shouldn't cause you any problems, when run within specs.