I have posted this question in an existing thread on the main forum as well. Would there be any downside to using geofoam with radiant on top under the slab?

Geofoam is just EPS (Expanded Polystyrene) which is commonly used under the slab.

We have seen geofoam come in both EPS and XPS. Geofoam typically comes in blocks and not in sheets. As such, geofoam is typically a couple feet thick instead of a couple inches thick. If this is truly what you are stapling your PEX on, you will not experience any significant degradation in the slab heat transfer efficiency. However, if you only have a couple of inches of foam under the concrete and you really want to maximize the slab heat transfer efficiency, you may want to get the PEX at least into the middle of the concrete.