Shane, if an exothermic nuclear reaction that can often and reliably generate 20-100% overunity exists, as claimed here, it is always going to be possible to generate an unambiguous experimental result, especially because nothing about nuclear mechanisms makes the output power directly depend on the input power. So there is no inherent limit on COP.
Currently known nuclear reactions don't depend on molecular diffusion. Proposed LENRs are likely to. IMO.