what the PWM does is control the flow rate up/down to maintain the DT , at lower output levels when running in a steady state for a long period. For a given lower output, you can have either the flow rate staying the same (Fixed speed pump) and DT reduced, or the DT can stay the same and the flow rate reduces (variable speed pump).

there is an FSV , can't remember the number of the top of my head, for Target DT. with a PWM pump in the system, the samsung will vary the flow rate to try to maintain this DT but only when its in the stable part of a heating cycle. It has no effect when doing DHW, or when in the initial startup part of a heating cycle (Which is 30-60 mins , at least on my system). This FSV for DT control method  has no function if you don't have a PWM pump (this is a common misconception), and is of little use in a system with primary/secondary seperation.

definitely. Bigger rads and lower flow temperature is your low hanging fruit. 

radiator changes - yes - put your effort in there first. obvious gains as you have found.

Also I just swapped out a small radiator for a much bigger one in my kitchen which resulted in a jump of daily COP from 3.6 to 4.1, I have three other rads I want to swap as well when I can afford to do it. I think thats probably a first step before removing a LLH or changing pump. 

lots of run data on my heatpumpmonitor.org data, as per my sig.

I managed to talk to Glyn Hudson who was having difficulties getting PWM to work with his Samsung. However he has managed to sort it with a firmware update of the indoor controller and now it works at low flow temperatures, so it seems Samsung have sorted it.

https://community.openenergymonitor.org/t/samsung-mim-c02n-s-net-pro-dongle/25418/5