In a main campaign the enemy levels are hard-coded. That's not really a problem when playing the main campaign. However, if you create a quest mod giving you 3 or more levels then your characters becomes over-leveled. Even if you would skip some non-essential battles in the main campaign it would still take some time until the over-leveling becomes re-balanced. So, XP giving quest mods for the main campaign do not work very well in this game.
I was thinking about that I could convert the UnderSiege into a stand-alone mod without a main dependency. But, who is interested about a short stand-alone mod ? The mod is finished before you are even properly warmed-up. Well, I suppose it should be possible to mod a mod. But is anyone interested in modding the UnderSiege and creating additional quests for it or even a next chapter ? Or, would some other mod be a better core-mod ?