An over flow box will keep the top layer of water in your tank turning over. Just a drain will not skim the top layer aross the whole area of the tank. It will pull water from just in front of the bulkhead fittings. An not evenly across the whole surface like and overflwo box.
The other thing to keep in mind is you’ll need additional space in your sump. In the event of power loss, water will drain to the lowest point of bulkhead/strainer. Whereas with the overflow box, it will only drain the water above the teeth/overflow wall, and obviously what’s left in the box. Trust me…there is a big difference!!!