Here's maybe another way to think of an overflow system: Imagine a 10 gallon tank full of water - nothing else. you now take a cup of water and pour it into the tank - what will happen? It will overflow of course, but only until the quantity of water you just added has overflowed - it will stop overflowing once it is back to 'full' again. If I don't add another cup of water it will just stay full - if i do add another cup then it will overflow again.
I now place the tank of water over an empty tank so that when I poor water into the tank it overflows into the lower tank. I fill both tanks with water. If I take a cup of water out of the bottom tank it is no longer full, but if I pour it into the upper tank, this tank overflows to fill up the lower tank - no water has enterred or left the system, it has just been moved by me with a cup - this is a 'closed loop' system.
I now get rid of the cup and place a pump into the lower tank (not running). I have a length of tube attached to the outlet of the pump and I put the end of this into the upper tank. The pump is not running, so nothing really happens, I still have two tanks full of water and that is it.
I now switch on the pump. It moves water from the lower tank to the upper tank (as I did with the cup - but it does it continuously). Whatever water it moves to the upper tank overflows back down to the lower tank, and so it continues.
So what happens if I turn off the pump? = no water gets moved to the upper tank, so no water overflows to the lower tank and once again I have two full tanks.
To get back to your question: only as much water will overflow as is being pumped up there. so if I used a strong (high flow rate) pump, then more water would be moved up and consequently that same 'more' water would overflow. If I used a lower powered pump, less water would be moved up and thus less water would overflow.
So in the end it balances itself out perfectly :-)
More confused or did that explain it?