By the look of it, most people are running fixed speed pumps (ie fixed flow rates).
So the only way you will get 5kW+ out of the heat pump is by adjusting the DT.
Less heat output = tighter DT
More heat output = wider DT
Something like this
Hope that makes sense?
So the problem with dropping the flow rate, is that when it gets cold and you need 5kW, you can’t get there. The maths don’t add up, unless you really go very wide on the DT.
If and when the cold snap happens, the maths suggests we should see the DT start to widen.
PS. SHC = Specific heat capacity (SHC), Water is 4.2 and it’s 3.8 for Gylcol.
(as glycol isn’t as good at transferring heat, so heat pump has to work harder to get the outputs)