There is a Watchlist preference for "Reload the watchlist automatically whenever a filter is changed." This preference has no effect when the New Filters are active, since this is how the New Filters work anyway. So we should hide it when New Filters are active (either because the New Filters beta is selected or because we have graduated New Filters out of beta).
The preference should be stored behind the scenes and reinstated for the user if she de-activates New Filters again.