The abby demon v dust question is an interesting one, and it goes to show why we restatted monsters in the early/mid game (and is something i want to continue doing)
E.g. Abyssal demons should likely have more HP than they do being such a high-level mob - but being a commonly killed monster would mean they also need drop table adjustments to make up for more HP as you stated.
I don't personally think the existence of a slayer Req on a monster should instantly boost its XP values, because it tends to have benefits (e.g. unique drops / being able to gain slayer XP at the same time)
That being said post-launch, I may look into adding XP mods to some monsters (things that come to mind are things like mammoths that force you into 1v1, or are slightly more awkward to AoE etc.) To make up for the changes (though, they weren't remotely competing with something like abyssal lords previously, they could be in a better spot.)
Essentially the combat XP system wasn't in a good enough state to support Necro's release, so we simplified it, but it won't be the end of the changes, and monsters (xp and stats) I'm going to keep looking at post-launch.