From experience with AQ40 on Anathema, the numbers returned by the UnitResistance API call are incorrect. On some bosses like Ouro, where the API call says he has 0 Fire Resistance, I still encountered a lot of partials. Even though I've been told on the developer discord to trust the API call response over the database values, I still think the database values for spell resistances are the ones applied on the server. For AQ40, these look like the following (With curses already taken into account):
My copy of the database has had the same migrations applied as the ones that are active on the Anathema realm.
EDIT: I have tried using a variation of spell penetration pieces without getting very conclusive results as I am not recording my fights.