A power head will create a water stream / current that helps to circulate the water.
I honestly do not think a sponge filter will contribute to more flow regardless putting it on the opposite corner of your HOB. Aeration is associated with water agitation which is important for gas exchange. A power head will create a water stream / current that helps to circulate the water. If you can, then put the HOB filter on the side of the aquarium to send the flow across the other side of the tank rather than on the back where the flow is directed towards the front of the glass and disperses the flow from there.