Typically overflow GPH isn't really figured into tank turnover calculations - you want 3-5x tank volume moving through the sump/fuge, more isn't really necessary and can sometimes cause issues (such as bubbles, or possibly what you're experiencing). I'd use powerheads or a closed loop to provide more flow if it's needed, and try slowing the amount of water moving through the sump. If the overflow is better able to handle the amount of water going through it, your surface scum issue should pretty much clear up immediately
