Free halimeda!

[Delphinus]

Nope ... an overflow box will not siphon out water faster than the sump return. In an overflow box design there is a box on the inside of the tank and a box on the outside of the tank, connected by a U-tube. The pivotal key of the design is the dam in the outside box. Water has to rise over the dam in order to drain down into the sump. It can only flow over that dam at whatever rate is it being pumped back into the main display tank.

Maybe it's easier to explain with the overflow box actually in hand! It can be hard to visualize based on textual descriptions..