I think you may misunderstand me.
I suspect that you may find a trickle of hot water coming out of the open vent (OV) pipe and splashing into the feed and expansion cistern (pumping over) when the pump is running. The arrangement of the OV and CF connections make this possible. It sometimes only happens at part load, when many of the TRVs have throttled down.
Some of the water takes a short cut, up the open vent and down the cold feed, instead of flowing around your heating circuit. This is a problem, not because of the loss of flow/pressure, but because the water is aereated by the churning and splashing and absorbs oxygen from the atmosphere. It carries this dissolved oxygen into your radiators, where the oxygen rusts the steel inner surfaces, turning it into black magnetite sludge.
The magnetite sludge circulates in your pipes, settles in the pipes, blocks valves, block radiators (causing cold spots), blocks 10mm pipes and can wreck a boiler, especially newer high efficiency boilers, when it blocks the narrow waterways. This is possibly the root cause of your problems. If it is pumping over, you need to sort out the F&E tank so that it stops producing magnetite, and try to remove the magnetite sludge already in the system (flushing, power flushing, magnaclean or similar, etc ) as a first priority.
Rearranging the CF & OV connections would probably stop it happening; converting to a sealed system, if practical, certainly would. There could be other problems ( e.g., leaks that require regular topping up, weakened radiators that fail under the higher pressure). I'd go for a sealed system. There's information on conversion kits on the RWC site, I think).
You may need that, but sort out any pumping over problems first.
You may find the existing pump can cope once some of the sludge has been removed. Or there could be other problems.
