You’re both right. The end result of the calculation (heat output) is the same regardless how you manipulate the DT, flow rate and SHC.
BUT and this is the big but as Daniel alludes to. The Arotherm works by fixing the flow rate and modulates by adjusting the DT. Narrow DT when it needs low output and opens the DT up when it needs more output. There isn’t really anything to tweak unless you start dropping flow rate.
Personally, I would not get obsessed with DT5 at the radiators. The outdoor unit will still do DT2 when low output.
I got obsessed with DT5 at the rads and ended up closing down my rads so much that I dropped my system volume and ruined my COP performance.
Aim to open all the rads as much as possible so you have as much system volume as possible, but at the same time, the rads need to be closed down just enough to ensure resistance so that all the rads in the circuit get some heat. In my case, fully opening up every rad meant my index rads (the ones furthest away) didn’t get much heat.
Once you’ve got heat to every rad, you can tweak the lockshield slightly to adjust room temp.
So it’s a balance!! (no pun intended).