[MetalMan]

The BEC on the MGM controllers can handle 4s Lipo. Where you by chance using it in a monster truck? Because the BEC circuit can only handle so many watts, because it converts the wasted energy into heat. The equation to figure out the watts that need to be turned into heat is:
(input voltage - output voltage) x amps pulled
So in your case: (~14.8v-5/6v) x amps that the servo pulls. Let's say that the servo pulls 1 amp. This gives you 9-10watts of heat. That's quite a bit for something that can't displace the heat. And if you are using a heavy monster truck, a servo in that can easily pull 3amps. So, up to 30watts! Sinking 30watts requires quite a large heatsink, and will also require quite a good fan!