So to be clear an EASA medical is sufficient to exercise the privilege of an FAA IR (Test Passed) add-on to my part 61.75 PPL.
That's correct. The FAA do not require any change in medical certification for IR privileges and there is no explicit limitation on a Class 2 EASA medical that restricts it to VMC (an audiogram is required for an EASA IR but not required by the FAA).