Ecocirc Curve below has the same performance at 6M as most other 6M pumps like the Alpha3.
If you assume tat the 16rads output at a total of 24kw, a rad dT of 15C requires 1LPM so total flow is 24LPM or 1.44m3/hr, say 1.5m3/hr.
Your own pump + the two below all give this flow at ~ 4.0/4.1M.
My only slight concern/suspicion is that the Ecocirc is actually a 6M pump and they achieve a 8M head simply by increasing the RPM, now if this is correct then it may be running at the same speed (at 6M) as your original pump (or even higher) but hopefully won't result in the same problem, Of course the same might apply if you had choosen the Alpha 3 or any other 6M pump.
You may not require a 4/4.1M head so CP or PP settings will result in reduced speed to give the appropriate head if any problems arise.
Ok I think I follow that! I guess my main worry with the Grandfos is that it was just seemed to be three set speed settings. I may be wrong but I did email Grandfos enquiring about this and if they could recommend one of their models. Whilst they did reply, it wasn't massively helpful and didn't answer the question. They just said as my existing pump was heating up the rads at top speed then the problem was my pipes....
I don't think you will have any problem with the new pump.
You will often see gas boilers giving a max flowrate based on a 20C boiler dT, your 28kw might have a max flowrate based on this of, 28X860/60/20, 20LPM, 1.2m3/hr and they may also give the pressure drop through the heat exchanger which can often be 2 to 3M so the boiler circ pump has very little remaining head to circulate the water around your system so they generally recommend the installation a low loss header (LLH) with its own pump on the secondary side, the second pump then has plenty of head to circulate the heating system or/and HW coil. Again, some gas boilers have much lower Hx pressure drops than others so you may have little trouble in just using one circulation pump.
Here is a example of a 30kw Glowworm Ultracom HXI which has a dP of 2.4M at 1220LPH, 20LPM, 1.2m3/hr so you can see why 6M pumps can struggle trying to then circulate through the rads after loosing 2.4M through the Hx. An oil fired boiler might only loose 0.25/0.5M through its Hx at this flow.