if you get an overflow rated at 600gph, well if its on a 29g tank, it might not siphon that much if the 600gph is rated on a 75g tank. The pump is what makes the difference. Once it gets below the overflow line, its the pumps job to keep the water above that line.
"On a 55 gallon tank with mag5 pump you will get a 500gph flow rate, on a 75 gallon you could get a flow rate of 700gph with a mag7 pump, and on an aquarium with a larger surface area and a larger pump you could get a larger flow rate, and with a smaller surface area and smaller pump you will get a smaller flow rate."
This was taken from
Wet/Dry Aquarium filters and overlfow boxes