what size is you drain piping for the overflow? If you have a 1 1/2" bulkhead you should be able to handle more than 1200 gph.
If what is limiting you is the size of the overflow box, you could always widen the size of the gaps between the teeth or simply remove the teeth and use gutter guard mesh to prevent things from going in the overflow.
This will increase the volume of water the overflow box can handle