I agree I read somewhere that the balance is about 1/3 radiative to convective on a hot radiator.
Clearly with the more fins and cooler temperatures the proportion of radiated heat will be even less.
On something like a warm floor or wall i suspect that the proportion of radiative heat might be quite a bit more. Although the temperatures are lower the areas are also much greater. The convection will also be less effective as much of the upper half of the wall will be trying to heat air that is already warm and flowing up past it!