I don't understand the first part of your post... The way an overflow is always supposed to work is that the return pump is pushing less water than the drain (in your case overflow) can handle. Why would you "cut back" the flow from the overflow?
You should never restrict the overflow because restrictions have a way of catching debris and becoming more restrictive without you knowing about it. This almost always results in the tank overflowing and the floor getting damp.
The only scenario I can think of is that your sump was at risk of overflowing with the overflow "wide open", which means that your sump 100% would overflow in the event of a power outage (you need to maintain a lower "normal sump level").
Now, if your overflow is flowing at less than it's rated volume, something is blocking normal flow. It's my understanding that the CPR overflows require a venturi pump to keep a bubble from forming in the siphon (I think the CPR is a bad design and I normally recommend lifereef overflows). If the venturi has stopped working, a bubble could have formed, which would slow and eventually stop the overflow as it grew. You also need to check your drain plumbing for growths of algae, feather dusters, and/or trapped snails and trapped algae. It is amazing what will grow in the nooks and crannies of an aquarium system.
I hope this helps,
Ross
