There are a few people who have ran all disc priest comps in this expansion, to varying degrees of success. Out of the ones I am aware of, only Ebony post on these forums, so she might have better insight in to this. As far as just questing, or some casual WPVP, they should do OK if you can manage them well enough. I however highly doubt you'd be able to handle high level instances all too well. I don't see them surviving the pressure that a tank would usually take in Mythic+ I'd love to be proven wrong, but I am skeptic about that happening.