Technically neither home "radiators" nor cylinder heads and hydros use heat radiation as the main method of heat transfer. There may be some heat radiation but it's negligible. The two mechanisms of heat transfer are conduction and convection. Since the conductivity of paint is lower than that of bare metal and surface area of bare metal will be greater than the smooth surface of paint, a bare metal surface will transfer heat more efficiently than a painted surface. By how much, I don't know.
Some may provide anecdotal evidence to support their position on the matter but to really get any kind of meaningful information you would need to do a serious study and measure the transfer rates in both situations. Should such a study reveal a large difference, say 20% , you could begin to talk about possible significant effects such as overheating. If it were to reveal a 2% difference, it would be hard to attribute engine overheating to that 2% , there would have to be another major underlying factor.
In the absence of exact data, and knowing that bare metal will transfer heat more efficiently (how much more I don't know), I would suggest leaving the surface unpainted.
Should the data become available and indicate a negligible difference, I would then suggest painting since it would provide protection from corrosion.
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