Ross Cameron asks a great question: what distinguishes legitimate from illegitimate appeals to the claim that "metaphysical priority isn’t epistemic priority"? To illustrate, consider Lewisian modal realism: although the Lewisian takes the possibility of talking donkeys to consist in the concrete existence of another spatiotemporally isolated world in which there is a talking donkey, he can claim to know the concrete fact in virtue of his prior knowledge of the modal one. On the other hand, it would be absurd for a Utilitarian to analogously argue that his prior moral knowledge that murder is wrong can ground his non-moral belief that murdering Bob would reduce unhappiness in the long run. Such reasoning would seem backward: in this case metaphysical and epistemic priority must coincide. Why the difference?
One potential difference that jumped out at me is that the plausibility of Lewisianism (but not utilitarianism) seems to depend on the concrete facts being a certain way.
That is, if an oracle tells us that ours is the only concrete world, Lewisians will (presumably) abandon their view rather than conclude that talking donkeys etc. are impossible. Lewisian realism requires a plenitude of concrete worlds in order to achieve its theoretical ambitions. (If there aren't enough concrete worlds, then the Lewisian won't be able to construct enough propositions, properties, etc. We'd need to give up Lewisianism and turn to abstract possible worlds to finish the job.)
Not so for utilitarianism. If we learnt that various acts of terrorism or murder actually promoted happiness in the long run, utilitarians wouldn't blink. They'd just conclude that those particular actions were objectively right after all (though perhaps not rational or 'subjectively right' given the antecedently available info). The theoretical motivation for utilitarianism does not at all depend on how the concrete facts turn out. It's compatible with the whole variety of conceivable outcomes.
This difference would seem to explain the noted epistemic asymmetry. Utilitarians must give some credence to the possibility that a given murder will be beneficial (compatibly with their theory being true), whereas Lewisians should not give any credence to there being few concrete worlds (compatibly with Lewisian realism) -- if they accepted the view about concreta, they would reject a Lewisian account of their modal significance. This explains why modal, but not moral, beliefs can appropriately be taken as 'epistemically prior' to (and unrevisable in light of) the concrete facts that (putatively) metaphysically ground them.
(1) Modal beliefs are of course revisable in light of other considerations (specified in our modal epistemology). There might even be some exceptions to their proposed unrevisability in light of oracular pronouncements about concreta. For example, suppose the oracle tells us that there is indeed a plenitude of concrete worlds, roughly corresponding to everything that strikes us on reflection as genuinely conceivable, but with one exception: no talking donkey worlds. If this is the only apparent 'gap', a Lewisian might take the relative plenitude as adequate confirmation of his view, and thus take the lack of talking donkeys as a kind of higher-order evidence that we'd made some procedural error in the conduct of our ordinary moral epistemology. For example, if ordinary moral epistemology rests largely on conceivability considerations, then the Lewisian might take this as evidence that talking donkeys aren't genuinely conceivable (hence possible) after all.
But that's a weird case. Even if a Lewisian's modal beliefs are sometimes revisable in this weird and circuitous fashion, it doesn't undermine my core claim that there's a general difference between the Lewisian and Utilitarian theories in this respect -- a difference that can be explained by the fact that claims about concreta are more apt to undermine Lewisianism than Utilitarianism.
(2) There may be other salient differences between these theories. For example, whereas Lewisian modal realism metaphysically reduces the modal to the concrete, we may (once we distinguish the property of being a right-making feature from the property of being right) doubt that normative theories provide the "metaphysical grounds" of moral facts, in this reductive sense. But does this difference between the respective 'grounding' relations play any role in explaining the noted epistemic phenomena? It's not immediately obvious to me, but further suggestions/explanations are welcome.
[Update: more here.]