I don't have a center overflow but that's only because there was already an back overflow on the tank, but the tank is 72" x 60" so if it was clear acrylic on all 4 sides I most definitely would have gone center overflow, even if at that size I could only see it from 3 sides (like it is now... because not many people have rooms that are that wide open)
That said, 12" x 12" on a 48" x 48" tank might have an odd feel to it. In the center that will leave just 12" of space between your overflow and your acrylic on all 4 sides over 12 of your 48 inches. Then combine that with what most people do with center overflows (which is cover them up with rocks and what not) then that reduces that space even more. Then you might want to think how you're going to get water flow moving, just returns? closed loops? Those small gaps you're creating with your overflow might cause some inconvenient dead spots.
Now I could see some interesting ways to make that work, where you basically get 4 smaller 18" x 18" cubes in each corner where tinier fish could have their own "territory", but still have space to move between areas. But I think if you want that grand majesty of a large tank a center overflow of that size might ruin it.
