Well Bubbles you have opened a can of worms here.
Yes I like my central over flow because it does open the tank right up. To see the over flows you have too get low and look up. They are not in the usual line of sight. You have to look for them.
If I was to do it again I would look at an external overflow box in the end corner if it was not a viewing end, other wise I would do what I have done. Mine is very noisy. Too much air you have to get rid of the air.
1) Same over flows - one as a safety for a full siphon system.
2) The second over flow would be full siphon with a tap on the outside, large tap and a pain to ase to tune(Cracker I took note, but had to do it my way, you know what I mean).
3) Other overflow hole could be small- full siphon tap if you don't want a safety, easier to tune.
A Cracker but external! - Also thanks Chris