You NEED your pump to pump slower than the overflows can go. That's the trick. Your overflows will only dump out anything that the pump can pump. If it were the other way, then the pump would pump too fast, your sump would empty out, and your main tank would spill over.
Let's say your overflows could do 1000 GPH
combined. You'd be safe with a pump anywhere up to 999 GPH
(theoretically, in a perfect algae free envt) AFTER accounting for head pressure. 5' of head pressure could bring down your GPH
rating by 200+ GPH
. My mag 9.5 can't even come close to my two overflows... which is good.
You always want your overflows & tubes to be clean & free from barriers. Clean them out regularly so that your flow is optimal.