are the cables in free gas ? - I'd expect they might also cool via being clipped to something metal ) The hard part is the circulation of the gas if you are relying on it and how it will convect and shed its heat to the enclosure.
If there is scope to do so under run by one size and give it no further thought but such things certainly can be calculated but it is not easy -  there is specialist 3d heat flow software for this sort of thing, and it is thousands of points per man month, so for a small project a quick test is likely to be easier
mike

are the cables in free gas ? - I'd expect they might also cool via being clipped to something metal ) The hard part is the circulation of the gas if you are relying on it and how it will convect and shed its heat to the enclosure.
If there is scope to do so under run by one size and give it no further thought but such things certainly can be calculated but it is not easy -  there is specialist 3d heat flow software for this sort of thing, and it is thousands of points per man month, so for a small project a quick test is likely to be easier
mike

The thermal conductivity of Argon is about half that of air at the same pressure, but that is not the only consideration as most heat from cables in an open space is lost by convection not conduction, and here the heat capacity of the gas is probably the most important, and Argon is higher than Air. Unless the cables are operating at close to the BS7671 ratings I would not worry too much, and the next larger size would be very safe, the ratings are very safe anyway, allowing for installation unknowns, and exact air flows. Don't forget the time factor of loading in your ideas, the thermal capacity of larger sizes makes them heat very slowly, so for modest periods the temperature will only climb slowly.  As usual the words "it all depends" are important!