If the convection efficiency is lowered by an insulating layer, the water returns hotter. I chose some ballpark numbers, under the (not really true) assumption that the heating is only convective:
Using dimensional analysis (because I don't remember the equation) I get
delta T (temperature drop across the paint thickness) in K:
[Thickness (m) * Power (W)] / [Area (m^2) Conductivity (W/mK)] = 1 (K) !
So a ballpark number would be a 1 degree K (or C) difference in surface temperature.
If the room is cold (10C) and the radiator is almost too hot to touch (140F = 60C) then it's a few percent effect on the convective efficiency, roughly speaking, and probably no more than 10%.
But a radiator temperature of 60C and room temperature of 10C, emissivity of 0.85, and area of 1 m^2 gives about 600 W of outgoing radiation, and ~360 W of incoming radiation from the room, or a net radiation of 240 W, so really can't ignore either one.