That seems a good tule of thump, I have a 16M2 room with one double rad, 1690 watts and it heats up in well less than a hour from cold (including system) the TRV then throttles in the flow and, depending on the OT, a output of ~ 350/550watts keeps it at 20C..My simple rule of thumb of 100 W per sq meter would make that rad output about right IF the flow temperature was about 82C or at its design temperature.
So are there any undisclosed heat losses? Leaking windows? Vents to the outside?
Kevin's room is 11.7M2 so the 1345watt rads should output almost 115watts/M2 at its rated output.
A T50 rad with flow/return temps of 75C/65C, dT 10C, will give its rated output with a room temp of 20C, it will also give its rated output with flow/return temps of 82C/62C, dT 20C, thats why its so important to know the flow/return temperatures, the actual and the rated output can then easily be calculated for any given flow temperatures, from that one set of readings.