NexGen has a power output of 160W/m². If the whole ceiling of a room 4m x 5m (i.e. 20m²) was covered, this would provide a total power of 20*160/1000 = 3.2kW.
Some of this power will be conducted upwards, some will be conducted downwards into the air, and some will be radiated into the room. Let's assume that the insulation above the ceiling is good enough that only 10% of the power is conducted upwards - so 90% (144W/m²) is left for heating the room.
If we assume a convective heat transfer coefficient of 0.95 W/m².K and a radiant heat transfer coefficient of 6.0 W/m².K, then the total heat transfer coefficient will be 0.95+6.0 = 6.95 W/m².K.
If the ceiling area is fraction K of the total room surface area, the ceiling temperature is Tc and the average temperature of the other surfaces is To, then the average radiant temperature of the whole room will be K.Tc + (1-K)To. If the air temperature of the room is Ta, the "operative temperature" (using a 50/50 weighting) will be ( K.Tc + (1-K)To + Ta) / 2. If Ta = To, then the operative temperature simplifies to ( K.Tc + (1-K)Ta + Ta) / 2 = (K.Tc + (2-K).Ta) / 2.
If the temperature of the rest of the room is about 20°C, then the ceiling temperature will be 20 + 144/6.95 = 40.7°C. If K = 0.25, then the operative temperature will be (0.25*40.7+1.75*20)/2 = 22.6°C.
If the temperature of the rest of the room is about 15°C, then the ceiling temperature will be 15 + 144/6.95 = 35.7°C. If K = 0.25, then the operative temperature will be (0.25*35.7+1.75*15)/2 = 17.6°C.
Suppose the heater power was increased to 280W/m² (5.6kW in a 20m² room).
If the temperature of the rest of the room is about 15°C, then the ceiling temperature will be 15 + 280/6.95 = 55.3°C. If K = 0.25, then the operative temperature will be (0.25*55.3+1.75*15)/2 = 20.0°C.
To look better, the NexGen should have a 5mm layer of plaster over it. The plaster will have an area heat capacity of about 6.5 J/m².K. So NexGen would initially raise the temperature of the plaster at a rate of 160/6.5 = 24°C/s, i.e. almost instantaneous.