OK, I've done a little bit of research. It would seem that the rating (in W) under British standards is for a delta-T of 50 deg C:
Now for a typical watercooling system in a PC, we would probably be looking at more like a delta-T of 15 deg C between room and water/radiator. Now comes the tricky bit - how to scale down the heat dissipated for the reduced delta-T. Digging around my old textbooks (it's been a long long time), I reckon that heat dissipation under natural convection in air is proportional to about delta-T^1.25 or so, assuming all other properties (height, area) of the radiator are the same. This would also be assuming that the heat transfer between water and steel radiator will never be our limiting factor... It is also consistent with the above page's mention of a move in the standard from delta-T of 60 to 50 deg C reduced heat outputs by just over 80%.
Thus for a delta-T of 15, I reckon a radiator rated at 500W would be good for just over 100W from the computer. Which should be good enough for my current CPU + GPU. And that's one of the smaller radiators too. So it could well be worth trying... and as they only cost about 20 quid nothing to lose really...
(apart from the cost and hassle of fittings etc.). Might well do so in a couple of months.