I see 'efficiency' as the wrong word, as clearly all heat energy in must come out as heat energy there can be no loses.
However I have been looking at the speed at which a radiator can heat a room. I looked at these
having had a proper Myson
fan assisted radiator and it worked well, the whole idea seemed good, so as an experiment I used a small fan and the thermostat normally used to brew beer to see if it helped.
However with a modulating boiler with lock shield valve and TRV head the experiment was a failure, sensor was on return, so the fan would only run if return water still warm, and the fan would switch off/on on a regular basis as to run the fan it needed more water through the radiator, the old Myson had no lock shield valve, and was used with a no modulating boiler, where hot water returning to boiler did not matter.
I have come to realise central heating is not as easy as it seems, back in around 1980 I fitted my first, at that time no TRV's and I found I had not allowed for the amount of heat which will peculate up from lower floor to upper floor. Up stairs too hot and down stairs too cool. The house before that had hot air central heating, blowing air around the room means whole room at same temperature, including at that time next to the single glazed picture windows so it increased the loses. But second house with double glazing fan assisted radiator worked well.
With a gas boiler the cooler the return water likely the more efficient the boiler runs, not sure with an oil boiler? In theory this house should cost more to heat than last one, as larger, and detached, where last one was a semi-detached, and double glazing is older, plus two sets of patio doors, but in fact the oil costs less than we paid for gas.
I have a fault I know, pump on return and pump hot before all radiators are hot, but pipe work hidden so don't know why. But since not a modulating boiler it still works.