The philosophy behind a base layer in heat is vaguely like the fins on a radiator: you are increasing the overall surface for heat transfer by making it three-dimensional.

On the other hand summer jerseys are already designed to do that so it is not clear to me how it is going to add much if you already have a mesh jersey that wicks well. And there is a trade-off of more fabric meaning more insulation and heat retention vs more fabric meaning more surface area for evaporative cooling.

So far I have not used a base layer in heat. If they were $5 I'd buy one and try it out. I recently watched some pro bike races in heat and recall seeing a lot more bare unzipped chests than unzipped jerseys with base layers showing.