I had the same problem when I first got my tank. The overflow that I paid to have made was not even close to being able to handle my Mag12 return (looked like it was designed for a Maxijet1200 return pump or something!).
The only way I could get the water level below the center brace was to cut off ALL the teeth, then grind another 1cm right off
I still have traces of black acrylic shavings in my tank. I'm now using a toothless overflow with an eggcrate screen. It works well enough.