Okay, i misunderstood your question due to term use...
'gravity fed' means that there is normally a hole at water level or on the base with an upstand, and water freely flows through the hole by gravity into the sump through a pipe. This is what most people have and I would call a gravity fed / overflow sump system.
I think you are asking about a syphon fed sump, that normally sits on the back of the tank and works via a syphon from one tank to the other and is returned by a pump.
You are correct in thinking that to balance a lower sump with a syphon is tricky, in fact, its pretty much impossible as one wrong move floods your floor!
OR, if i was initially right, like Randy in his post, that you are just wondering about water leaving your tank via a hole at a slower rate than its being pumped up then yes that is also a situation to figure out.
My cuttle tank for example had 3 holes drilled at water level and returned by a 802 powerhead, that worked fine... 3 because i was making sure that if one hole got blocked the other two would be easily able to cope...
hope you can settle this

have you seen a pic online to point to?