Although some quests have big XP optionals (A New Invasion comes to mind), the current system does not take into account that some optionals scale much more (or less) with difficulty level than the quest as a whole does.
As an example, heroic elite VON3 is much harder than heroic hard VON3, but the "Kill Chacter Luridae" optional is almost exactly the same difficulty on the two settings. Or for an example that is the other way around, Epic Normal Gianthold Tor is of a pretty comparable difficulty to Epic Hard, but the dragon optionals are considerably more difficult on EH than EN.
For this reason, I think heroic elite VON3's completion XP could be quite a bit more than heroic hard (perhaps 40% more), but Chacter Luridae should offer only 5% more at most, while the Tor dragons could do with quite significant XP increases on EE, EH and HE.
A proper pass of XP would likely consist of two Devs and/or GMs - one of whom has multiple TRed toons of their own, the other with no capped 36 pointers - spending 15 minutes discussing each quest and assigning it an XP value on the assumption that Siegebreaker is perfectly balanced. That's perhaps 16 total workdays (8 days each). Then put it all live, watch for a week, hotfix anything that you got badly wrong (e.g. if you gave Hold for Reinforcements a base XP of 44000 you'd want to patch that down to something more sensible like 18000), then review it again in three months.