Do any show the levels of quests?
-- No, I don't think this can be done yet.
Do any show how much xp you get from a kill?
-- XP can be shown in a 2000/2000 ratio. The actual XP numbers would eventually get to large (probably in the millions at higher levels) to be much use, but knowing a a specific mob gives you 2/2000 experience is a little easier to compare with than, say, a mob giving you 724/396758 experience.
Have more bars for actions/spells (all always visible without having to scroll through bars)?
-- I think 6 bars is the limit, but I'm not 100% sure. You can definitely change the layout and size of these.
Extra map features?
-- There is a program to add custom points of interest and probably some other stuff, if you check vgsohinterface.com
Are the mods pretty stable?
-- I would say yes. They are basically just graphical/layout changes. If there's anything wrong with them, it's probably due to the UI coding which would likely affect the default UI as well.