I ask because I've tried putting a sponge over the outtake spout and while it curbed the water flow issues, the heater couldn't work properly while in the little compartment where the outtake tube etc is.
I've heard a lot of people advocate piercing a number of holes in the outtake tube, which eases the flow while allowing more water circulation around the heater to help give a more even temperature.
Tho if you have your heater on the outside of the compartment, the sponge might work better for you than the holes method.
I drilled about 5 holes in the outtake tube and put the heater in the compartment with the pump. I haven't noticed any issues keeping my Spec V to temp.
My betta lives in the tank and I just turn the output nozzle toward the glass wall. That made the flow manageable for him. You could put a filter sponge over the output but, for me, it dispersed the flow too much and created some dead zones in the tank.