I done some calculations, and reckon that 4 fan units provide an extra 330 W of heat output at Δ20, an increase of 25% on those radiators. This seems in line with independent testing that SpeedComfort mention on their FAQ page, if you extrapolate to the right towards 40C:
[Power increase in % vs. Supply water temperature in °C]
Correlating to some measurements I did of the whole system, this increases the total effective output at Δ50 by 1,500 W (but not real output at that temperature), and drops the overall flow temperature by 0.5 C.