Don it really depends on the tank. If you are going to have a softie tank lower flow is encouraged (to a Point). For most tanks however you want a good deal of flow and flow that covers the tank. In running this kind of flow through your sump your going to have issues with bubbles as the water is moving to quickly through it and they cant dispurse. Also most of the equipment you run in your sump needs good contact time, from say a heater to other auxilary devices.
On the second question, if it is open to air its not a closed loop. In the case of a bottom drilled overflow it will be subjected to bubbles as the water comes over the top of it, these bubbles usually get compressed as the go through the CL and create a mess in the tank.
Their are a few tricks to make an overflow a true cl if you want though
Mike