If you are looking to buy an overflow, this one looks ok. I got one from aquacrylics.com trough ebay. you can get them straight from their site. It is similar in design. It claims to get 700 GPH
with the correct pump. In two weeks of repeated trials, I have never broken the siphon, and I turn it off during feeding times everyday also!
You are correct, if you break the siphon it will overflow your main tank, but all (J tube) overflows should be designed with a baffle that keeps water in them, so the siphon starts again as soon as the pump starts.
Because I am paranoid about this I am still thinking about a battery backup that would kick in if the power were to go out.