Two standards that apply:
ASTM D 1621 — Standard Test Method for Compressive Properties of Rigid Cellular Plastics.
ASTM C 165 — Standard Test Method for Measuring Compressive Properties of Thermal Insulations.
16 to 25 psi (110 to 172 kPa) is the common range of polyiso compression strength. I've been walking around on 2 3/4" polyiso lately with no issue. With plywood over top I'm certain you'd be fine.
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