I prepared for EB1b, but didn't get a chance to really .
When I read the USCIS deion and same sample letters, especially the deion from the I-office of Duke U., it will be the best if the recommendation letters approves that you are truly outstanding in such and such areas...
My understanding is that for EB1b (Outstanding Researcher), you need to claim that you are outstanding. No only you say it yourself, you also need other senior person in your fields to attest that for you.
So the support letters should be emphasizing your contributions in the areas (which still could be national interest, but for eb1b use that to approve your outstanding ability and potential etc.).