I have used those servos. I believe you mean ST125 MG sport servos with 125 oz/in.

First, they are horrible servos. They center poorly and are very sloppy. I no-longer use them.

BUT.... they don't really drain much more power than a standard servo unless you are really banging them around.

I would make the jump to lithiums now and run a 7.4V 2400 MAH Fromeco pack and a regulator. It will last for days in that plane and it can be used later in other planes. You will save some weight too.