I did experiment with a soil temperature probe and found that they were more stable than pots (the surface of which can get quite hot in direct sun). I don't keep my seedling tubs in direct sun, though.
Of course you're in a much warmer climate than mine. I watch your various posts and you are about a month ahead of me. Also with the UV index being such a major issue in the southern hemisphere there's much more radiation to consider ( including that which causes UV deterioration of the styrofoam). However the temperature insulating properties are good plus to consider. As it would generate a much more stable temperature for the substrate/medium, the benefit for the seedlings become obvious. If UV deterioration is the only down, here I would wrap it with left over greenhouse lining that still has the UV filter but is no longer suited for its original purpose because it has lost transparency. If were closer I would gladly provide you with some for free
...I'm now devising ways of how can I best use this leftover / used material after its designed life expectancy . Now I'm using it underground as impermeable lining for my very dry sandy soil garden beds ,following the xeriscape gardeners' ideas tested in NMexico or Arizona ( US )where there is a very strict water rationing for irrigation or leisure purposes ( swimming pools).
Since lilies also demand good moisture whilst we become water restricted, then I'm going to add it to my future lily beds
Arturo